MAX1400CAI ,%V, 18-Bit, Low-Power, Multichannel, Oversampling Sigma-Delta ADCFeaturesThe MAX1400 18-bit, low-power, multichannel, serial-' 18-Bit Resolution, Sigma-Delta ADCout ..
MAX1400CAI+ ,+5V 18-Bit Low-Power Multichannel Oversampling (Sigma-Delta) ADCApplicationsCS 3 26 DOUTPortable Industrial InstrumentsRESET 4 25 INTPortable Weigh ScalesMUXOUT+ 5 ..
MAX1400CAI+ ,+5V 18-Bit Low-Power Multichannel Oversampling (Sigma-Delta) ADCELECTRICAL CHARACTERISTICS(V+ = +5V ±5%, V = +2.7V to +5.25V, V = +2.50V, REFIN- = AGND, f = 2.4576 ..
MAX1400EAI+ ,+5V 18-Bit Low-Power Multichannel Oversampling (Sigma-Delta) ADCFeaturesThe MAX1400 18-bit, low-power, multichannel, serial-♦ 18-Bit Resolution, Sigma-Delta ADCout ..
MAX1402CAI ,+5V / 18-Bit / Low-Power / Multichannel / Oversampling Sigma-Delta ADCfeatures matched' 16-Bit Accuracy with No Missing Codes to 480sps200µA current sources for sensor e ..
MAX1402CAI+ ,+5V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADCELECTRICAL CHARACTERISTICS(V+ = +5V ±5%, V = +2.7V to +5.25V, V = +2.50V, REFIN- = AGND, f = 2.4576 ..
MAX4018ESD ,Low-Cost / High-Speed / SOT23 / Single-Supply Op Amps with Rail-to-Rail OutputsApplications______________Ordering InformationSet-Top BoxesSOTSurveillance Video Systems TEMP. PIN- ..
MAX4019EEE ,Low-Cost / High-Speed / Single-Supply / Gain of +2 Buffers with Rail-to-Rail Outputs in SOT23applications that require wide bandwidth, such as-75dB Total Harmonic Distortionvideo, communicatio ..
MAX4019EEE ,Low-Cost / High-Speed / Single-Supply / Gain of +2 Buffers with Rail-to-Rail Outputs in SOT23applications, the MAX4014 comesin a tiny 5-pin SOT23 package. ' Low, 5.5mA Supply Current' 400µA Sh ..
MAX4019EEE+ ,Low-Cost, High-Speed, Single-Supply, Gain of +2 Buffers with Rail-to-Rail Outputs in SOT23applications, the MAX4014 comesin a tiny 5-pin SOT23 package. ♦ Low, 5.5mA Supply Current♦ 400µA Sh ..
MAX4019EEE+T ,Low-Cost, High-Speed, Single-Supply, Gain of +2 Buffers with Rail-to-Rail Outputs in SOT23Applications__________Typical Operating CircuitPortable/Battery-Powered InstrumentsVideo Line Drive ..
MAX4019ESD ,Low-Cost / High-Speed / Single-Supply / Gain of +2 Buffers with Rail-to-Rail Outputs in SOT23MAX4014/MAX4017/MAX4019/MAX402219-1284; Rev 0; 10/97Low-Cost, High-Speed, Single-Supply, Gain of +2 ..
MAX1400CAI
%V, 18-Bit, Low-Power, Multichannel, Oversampling Sigma-Delta ADC
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC19-1430; Rev 0; 2/99
General DescriptionThe MAX1400 18-bit, low-power, multichannel, serial-
output ADC uses a sigma-delta modulator with a digital
decimation filter to achieve true 16-bit accuracy. The
user-selectable decimation factor of the digital filter
allows the conversion resolution to be reduced in
exchange for a higher output data rate. The device
achieves true 16-bit performance at an output data rate of
up to 480sps. In addition, the modulator sampling
frequency may be optimized for either lowest power
dissipation or highest throughput rate. The MAX1400
operates from +5V.
This device offers three fully differential input channels
that can be independently programmed with a gain
between +1V/V and +128V/V. Furthermore, it can com-
pensate an input-referred DC offset (such as system off-
set) up to 117% of the selected full-scale range. These
three differential channels may also be configured to
operate as five pseudo-differential input channels. Two
additional, fully differential system-calibration channels
are provided for gain and offset error correction. External
access is provided to the multiplexer (mux) output to
facilitate additional signal processing.
The MAX1400 can be configured to scan all signal inputs
sequentially and provide the results through the serial
interface with minimum communications overhead. When
used with a 2.4576MHz or 1.024MHz master clock, the
digital decimation filter can be programmed to produce
zeros in its frequency response at the line fre-
quency and associated harmonics, ensuring excellent
line rejection without the need for further post-filtering.
The MAX1400 comes in a 28-pin SSOP package.
ApplicationsPortable Industrial Instruments
Portable Weigh Scales
Loop-Powered Systems
Pressure Transducers
Features18-Bit Resolution, Sigma-Delta ADC16-Bit Performance with No Missing Codes
to 480spsLow Quiescent Current
250µA (operating mode)
2µA (power-down mode)3 Fully Differential or 5 Pseudo-Differential Signal
Input Channels2 Additional Fully Differential Calibration
Channels/Auxiliary Input ChannelsAccess to the Mux Output/ADC Input Programmable Gain and OffsetFully Differential Reference InputsConverts Continuously or On CommandAutomatic Channel Scanning and Continuous
Data Output ModeOperates with +5V Analog Supply and
+3V or +5V Digital SupplySPI™/QSPI™-Compatible 3-Wire Serial Interface28-Pin SSOP Package
Pin Configuration
Ordering Information
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS(V+ = +5V ±5%, VDD= +2.7V to +5.25V, VREFIN+= +2.50V, REFIN- = AGND, fCLKIN= 2.4576MHz, TA= TMINto TMAX, unless other-
wise noted. Typical values are at TA= +25°C.)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
V+ to AGND, DGND.................................................-0.3V to +6V
VDDto AGND, DGND...............................................-0.3V to +6V
AGND to DGND.....................................................-0.3V to +0.3V
Analog Inputs to AGND................................-0.3V to (V+ + 0.3V)
Analog Outputs to AGND.............................-0.3V to (V+ + 0.3V)
Reference Inputs to AGND...........................-0.3V to (V+ + 0.3V)
CLKIN and CLKOUT to DGND...................-0.3V to (VDD+ 0.3V)
All Other Digital Inputs to DGND..............................-0.3V to +6V
All Digital Outputs to DGND.......................-0.3V to (VDD+ 0.3V)
Maximum Current Input into Any Pin..................................50mA
Continuous Power Dissipation (TA= +70°C)
28-Pin SSOP (derate 9.52mW/°C above +70°C)........524mW
Operating Temperature Ranges
MAX1400CAI.....................................................0°C to +70°C
MAX1400EAI...................................................-40°C to +85°C
Storage Temperature Range.............................-60°C to +150°C
Lead Temperature (soldering, 10sec).............................+300°C
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
ELECTRICAL CHARACTERISTICS (continued)(V+ = +5V ±5%, VDD= +2.7V to +5.25V, VREFIN+= +2.50V, REFIN- = AGND, fCLKIN= 2.4576MHz, TA= TMINto TMAX, unless other-
wise noted. Typical values are at TA= +25°C.)
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
ELECTRICAL CHARACTERISTICS (continued)(V+ = +5V ±5%, VDD= +2.7V to +5.25V, VREFIN+= +2.50V, REFIN- = AGND, fCLKIN= 2.4576MHz, TA= TMINto TMAX, unless other-
wise noted. Typical values are at TA= +25°C.)
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
ELECTRICAL CHARACTERISTICS (continued)(V+ = +5V ±5%, VDD= +2.7V to +5.25V, VREFIN+= +2.50V, REFIN- = AGND, fCLKIN= 2.4576MHz, TA= TMINto TMAX, unless other-
wise noted. Typical values are at TA= +25°C.)
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
Note 1:Nominal gain is 0.98. This ensures a full-scale input voltage may be applied to the part under all conditions without caus-
ing saturation of the digital output data.
Note 2:Positive Full-Scale Error includes zero-scale errors (unipolar offset error or bipolar zero error) and applies to both unipolar
and bipolar input ranges. This error does not include the nominal gain of 0.98.
Note 3:Full-Scale Drift includes zero-scale drift (unipolar offset drift or bipolar zero drift) and applies to both unipolar and bipolar
input ranges.
Note 4:Gain Error does not include zero-scale errors. It is calculated as (full-scale error - unipolar offset error) for unipolar ranges
and as (full-scale error - bipolar zero error) for bipolar ranges. This error does not include the nominal gain of 0.98.
Note 5:Gain-Error Drift does not include unipolar offset drift or bipolar zero drift. It is effectively the drift of the part if zero-scale
error is removed.
Note 6:Use of the offset DAC does not imply that any input may be taken below AGND.
Note 7:Additional noise added by the offset DAC is dependent on the filter cutoff, gain, and DAC setting. No noise is added for a
DAC code of 0000.
Note 8:Guaranteed by design or characterization; not production tested.
Note 9:The input voltage must be within the Absolute Input Voltage Range specification.
Note 10:All AIN and REFIN pins have identical input structures. Leakage is production tested only for the AIN3, AIN4, AIN5,
CALGAIN, and CALOFF inputs.
Note 11:The dynamic load presented by the MAX1400 analog inputs for each gain setting is discussed in detail in the Switching
Network section.Values are provided for the maximum allowable external series resistance. Note that this value does not
include any additional capacitance added by the user to the MUXOUT_ or ADCIN_ pins.
Note 12:The input voltage range for the analog inputs is with respect to the voltage on the negative input of its respective differen-
tial or pseudo-differential pair. Table 5 shows which inputs form differential pairs.
Note 13:VREF= VREFIN+- VREFIN-.
Note 14:These specifications apply to CLKOUT only when driving a single CMOS load.
Note 15:The burn-out currents require a 500mV overhead between the analog input voltage and both V+ and AGND to operate
correctly.
ELECTRICAL CHARACTERISTICS (continued)(V+ = +5V ±5%, VDD= +2.7V to +5.25V, VREFIN+= +2.50V, REFIN- = AGND, fCLKIN= 2.4576MHz, TA= TMINto TMAX, unless other-
wise noted. Typical values are at TA= +25°C.)
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
Note 16:Measured at DC in the selected passband. PSR at 50Hz will exceed 120dB with filter notches of 25Hz or 50Hz and FAST
bit = 0. PSR at 60Hz will exceed 120dB with filter notches of 20Hz or 60Hz and FAST bit = 0.
Note 17:PSR depends on gain. For a gain of +1V/V, PSR is 70dB typical. For a gain of +2V/V, PSR is 75dB typical. For a gain of
+4V/V, PSR is 80dB typical. For gains of +8V/V to +128V/V, PSR is 85dB typical.
Note 18:Standby power-dissipation and current specifications are valid only with CLKIN driven by an external clock and with the
external clock stopped. If the clock continues to run in standby mode, the power dissipation will be considerably higher.
When used with a resonator or crystal between CLKIN and CLKOUT, the actual power dissipation and IDDin standby
mode will depend on the resonator or crystal type.
TIMING CHARACTERISTICS(V+ = +5V ±5%, VDD= +2.7V to +5.25V, AGND = DGND, fCLKIN= 2.4576MHz; input logic 0 = 0V; logic 1 = VDD, TA= TMINto TMAX,
unless otherwise noted.) (Notes 19, 20, 21)
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
TIMING CHARACTERISTICS(continued)(V+ = +5V ±5%, VDD= +2.7V to +5.25V, AGND = DGND, fCLKIN= 2.4576MHz; input logic 0 = 0V; logic 1 = VDD, TA= TMINto TMAX,
unless otherwise noted.) (Notes 19, 20, 21)
Note 19:All input signals are specified with tr= tf= 5ns (10% to 90% of VDD).
Note 20:See Figure 4.
Note 21:Timings shown in tables are for the case where SCLK idles high between accesses. The part may also be used with the
SCLK idling low between accesses, provided CSis toggled. In this case SCLK in the timing diagrams should be inverted
and the terms “SCLK Falling Edge” and “SCLK Rising Edge” exchanged in the specification tables. If CSis permanently
tied low, the part should only be operated with SCLK idling high between accesses.
Note 22:CLKIN duty cycle range is 45% to 55%. CLKIN must be supplied whenever the MAX1400 is not in standby mode. If no
clock is present, the device can draw higher current than specified.
Note 23:The MAX1400 is production tested with fCLKINat 2.5MHz (1MHz for some IDDtests).
Note 24:Measured with the load circuit of Figure 1 and defined as the time required for the output to cross the VOLor VOHlimits.
Note 25:For read operations, SCLK active edge is falling edge of SCLK.
Note 26:Derived from the time taken by the data output to change 0.5V when loaded with the circuit of Figure 1. The number is then
extrapolated back to remove effects of charging or discharging the 50pF capacitor. This ensures that the times quoted in
the timing characteristics are true bus-relinquish times and are independent of external bus loading capacitances.
Note 27:INTreturns high after the first read after an output update. The same data can be read again while INTis high, but be
careful not to allow subsequent reads to occur close to the next output update.
Figure 1. Load Circuit for Bus Relinquish Time and VOLand
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
Pin Description
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
Pin Description (continued)
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
_______________Detailed Description
Circuit DescriptionThe MAX1400 is a low-power, multichannel, serial-
output, sigma-delta ADC designed for applications with
a wide dynamic range, such as weigh scales and pres-
sure transducers. The functional block diagram in
Figure 2 contains a switching network, a modulator, a
PGA, two buffers, an oscillator, an on-chip digital filter,
and a bidirectional serial communications port.
Three fully-differential input channels feed into the
switching network. Each channel may be independent-
ly programmed with a gain between +1V/V and
+128V/V. These three differential channels may also be
configured to operate as five pseudo-differential input
channels. Two additional, fully differential system-cali-
bration channels allow system gain and offset error to
be measured. These system-calibration channels can
be used as additional differential signal channels when
dedicated gain and offset error correction channels are
not required.
Two chopper-stabilized buffers are available to isolate
the selected inputs from the capacitive loading of the
PGA and modulator. Three independent DACs provide
compensation for the DC component of the input signal
on each of the differential input channels.
The sigma-delta modulator converts the input signal into
a digital pulse train whose average duty cycle represents
the digitized signal information. The pulse train is then
processed by a digital decimation filter, resulting in a
conversion accuracy exceeding 16 bits. The digital filter’s
decimation factor is user-selectable, which allows the
conversion result’s resolution to be reduced to achieve a
higher output data rate. When used with 2.4576MHz or
1.024MHz master clocks, the decimation filter can be
programmed to produce zeros in its frequency response
at the line frequency and associated harmonics. This
ensures excellent line rejection without the need for fur-
ther post-filtering. In addition, the modulator sampling
frequency can be optimized for either lowest power dis-
sipation or highest output data rate.
The MAX1400 can be configured to sequentially scan
all signal inputs and to transmit the results through the
serial interface with minimum communications over-
head. The output word contains a result identification
tag to indicate the source of each conversion result.
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
Serial Digital InterfaceThe serial digital interface provides access to eight on-
chip registers (Figure 3). All serial-interface commands
begin with a write to the communications register
(COMM). On power-up, system reset, or interface reset,
the part expects a write to its communications register.
The COMM register access begins with a 0 start bit.
The COMM register R/Wbit selects a read or write
operation, and the register select bits (RS2, RS1, RS0)
select the register to be addressed. Hold DIN high
when not writing to COMM or another register (Table 1).
The serial interface consists of five signals: CS, SCLK,
DIN, DOUT, and INT. Clock pulses on SCLK shift bits
into DIN and out of DOUT. INTprovides an indication
that data is available. CSis a device chip-select input
as well as a clock polarity select input (Figure 4).
Using CSallows the SCLK, DIN, and DOUT signals to
be shared among several SPI-compatible devices.
When short on I/O pins, connect CSlow and operate
the serial digital interface in CPOL = 1, CPHA = 1 mode
using SCLK, DIN, and DOUT. This 3-wire interface
mode is ideal for opto-isolated applications.
Furthermore, a microcontroller (such as a PIC16C54 or
80C51) can use a single bidirectional I/O pin for both
sending to DIN and receiving from DOUT (see
Applications Information), because the MAX1400 drives
DOUT only during a read cycle.
Additionally, connecting the INTsignal to a hardware
interrupt allows faster throughput and reliable, collision-
free data flow.
The MAX1400 features a mode where the raw modula-
tor data output is accessible. In this mode the DOUT
and INTfunctions are reassigned (see the Modulator
Data Outputsection).
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
Selecting Clock PolarityThe serial interface can be operated with the clock
idling either high or low. This is compatible with
Motorola’s SPI interface operated in (CPOL = 1, CPHA
= 1) or (CPOL = 0, CPHA = 1) mode. Select the clock
polarity by sampling the state of SCLK at the falling
edge of CS. Ensure that the setup times t4/t12and t5/t13
are not violated. If CSis connected to ground, resulting
in no falling edge on CS, SCLK must idle high (CPOL =
1, CPHA = 1).
Data-Ready Signal (DRDY bit true or IINNTT
= low)The data-ready signal indicates that new data may be
read from the 24-bit data register. After the end of a suc-
cessful data register read, the data-ready signal
becomes false. If a new measurement completes before
the data is read, the data-ready signal becomes false.
The data-ready signal becomes true again when new
data is available in the data register.
The MAX1400 provides two methods of monitoring the
data-ready signal. INTprovides a hardware solution
(active low when data is ready to be accessed), while
the DRDY bit in the COMM register provides a software
solution (active high).
Read data as soon as possible once data-ready
becomes true. This becomes increasingly important for
faster measurement rates. If the data-read is delayed
significantly, a collision may result. A collision occurs
when a new measurement completes during a data-
register read operation. After a collision, information in
the data register is invalid. The failed read operation
must be completed even though the data is invalid.
Resetting the InterfaceReset the serial interface by clocking in 32 1s.
Resetting the interface does not affect the internal reg-
isters.
If continuous data output mode is in use, clock in eight
0s followed by 32 1s. More than 32 1s may be clocked
in, since a leading 0 is used as the start bit for all oper-
ations.
Continuous Data Output ModeWhen scanning the input channels (SCAN = 1), the ser-
ial interface allows the data register to be read repeat-
edly without requiring a write to the COMM register.
The initial COMM write (01111000) is followed by 24
clocks (DIN = high) to read the 24-bit data register.
Once the data register has been read, it can be read
again after the next conversion by writing another 24
clocks (DIN = high). Terminate the continuous data out-
put mode by writing to the COMM register with any
valid access.
Modulator Data Output (MDOUT = 1)Single-bit, raw modulator data is available at DOUT for
custom filtering when MDOUT = 1. INTprovides a mod-
ulator clock for data synchronization. Data is valid on
the falling edge of INT. Write operations can still be
performed, however, read operations are disabled.
After MDOUT is returned to 0, valid data is accessed
by the normal serial-interface read operation.
On-Chip Registers
Communications Register
0/DRDY: (Default = 0) Data Ready Bit. On a write, this
bit must be reset to 0 to signal the start of the Com-
munications Register data word. On a read, a 1 in this
location (0/DRDY) signifies that valid data is available in
the data register. This bit is reset after the data register
is read or, if data is not read, 0/DRDY will go low at the
end of the next measurement.
RS2, RS1, RS0:(Default = 0, 0, 0) Register Select
Bits. These bits select the register to be accessed
(Table 1).
R/W:(Default = 0) Read/Write Bit. When set high, the
selected register is read; when R/W= 0, the selected
register is written.
RESET:(Default = 0) Software Reset Bit. Setting this
bit high causes the part to be reset to its default power-
up condition (RESET = 0).
STDBY:(Default = 0) Standby Power-Down Bit. Setting
the STDBY bit places the part in “standby” condition,
shutting down everything except the serial interface
and the CLK oscillator.
First Bit (MSB)(LSB)
Communications Register
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
FSYNC:(Default = 0) Filter Sync Bit. When FSYNC = 0,
conversions are automatically performed at a data rate
determined by CLK, FS1, FS0, MF1, and MF0 bits.
When FSYNC = 1, the digital filter and analog modulator
are held in reset, inhibiting normal self-timed operation.
This bit may be used to convert on command to mini-
mize the settling time to valid output data, or to synchro-
nize operation of a number of MAX1400s. FSYNC does
not reset the serial interface or the 0/DRDY flag. To clear
the 0/DRDY flag while FSYNC is active, simply read the
data register.
Global Setup Register 1
A1, A0:(Default = 0, 0) Channel-Selection Control Bits.
These bits (combined with the state of the DIFF, M1,
and M0 bits) determine the channel selected for con-
version according to Tables 8, 9, and 10. These bits
are ignored if the SCAN bit is set.
MF1, MF0: (Default = 0, 0) Modulator Frequency Bits.
MF1 and MF0 determine the ratio of CLKIN oscillator fre-
quency to modulator operating frequency. They affect
the output data rate, the position of the digital filter notch
frequencies, and the power dissipation of the device.
Achieve lowest power dissipation with MF1 = 0 and MF0
= 0. Highest power dissipation and fastest output data
rate occur with these bits set to 1, 1 (Table 2).
CLK:(Default = 1) CLK Bit. The CLK bit is used in con-
junction with X2CLK to tell the MAX1400 the frequency
of the CLKIN input signal. If CLK = 0, a CLKIN input fre-
quency of 1.024MHz (2.048MHz for X2CLK = 1) is
expected. If CLK = 1, a CLKIN input frequency of
2.4576MHz (4.1952MHz for X2CLK = 1) is expected.
This bit affects the decimation factor in the digital filter
and thus the output data rate (Table 2).
FS1, FS0:(Default = 0, 1) Filter Selection Bits. These
bits (in conjunction with the CLK bit) control the deci-
mation ratio of the digital filter. They determine the out-
put data rate, the position of the digital filter-frequency
response notches, and the noise present in the output
result. (Table 2).
FAST:(Default 0) FAST Bit. FAST = 0 causes the digi-
tal filter to perform a SINC3filter function on the modu-
lator data stream. The output data rate will be
determined by the values in the CLK, FS1, FS0, MF1,
and MF0 bits (Table 2). The settling time for SINC3
function is 3 [1 / (output data rate)]. In SINC3mode, the
MAX1400 automatically holds the DRDY signal false
(after any significant configuration change) until settled
data is available. FAST = 1 causes the digital filter to
perform a SINC1filter function on the modulator data
stream. The signal-to-noise ratio achieved with this filter
function is less than that of the SINC3filter; however
SINC1settles in a single output sample period, rather
than a minimum of three output sample periods for
SINC3. When switching from SINC1to SINC3mode, the
DRDY flag will be deasserted and reasserted after the
filter has fully settled. This mode change requires a
minimum of three samples.
Global Setup Register 2
SCAN:(Default = 0) Scan Bit. Setting this bit to a 1
causes sequential scanning of the input channels as
determined by DIFF, M1, and M0 (see Scanning
(SCAN-mode) section). When SCAN = 0, the MAX1400
repeatedly measures the unique channel selected by
A1, A0, DIFF, M1, and M0.
M1, M0:(Default 0, 0) Mode Control Bits. These bits
control access to the calibration channels CALOFF and
CALGAIN. When SCAN = 0, setting M1 = 0 and M0 = 1
selects the CALOFF input, and M1 = 1 and M0 = 0
selects the CALGAIN input (Table 3). When SCAN = 1
and M1 ≠M0, the scanning sequence includes both
CALOFF and CALGAIN inputs (Table 4). When SCAN is
set to 1 and the device is scanning the available input
First Bit (MSB)(LSB)
First Bit (MSB)(LSB)
Global Setup Register 2
Global Setup Register 1
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADCchannels, selection of either calibration mode (01 or 10)
will cause the scanning sequence to be extended to
include a conversion on both the CALGAIN+/
CALGAIN- input pair and the CALOFF+/CALOFF- input
pair. The exact sequence depends on the state of the
DIFF bit (Table 4). When scanning, the calibration
channels use the PGA gain, format, and DAC settings
defined by the contents of Transfer Function Register 3.
BUFF:(Default = 0) The BUFF bit controls operation of
the input buffer amplifiers. When this bit is 0, the inter-
nal buffers are bypassed and powered down. When
this bit is set high, the buffers drive the input sampling
capacitors and minimize the dynamic input load.
DIFF:(Default = 0) Differential/Pseudo-Differential Bit.
When DIFF = 0, the part is in pseudo-differential mode,
and AIN1–AIN5 are measured respective to AIN6, the
analog common. When DIFF = 1, the part is in differen-
tial mode with the analog inputs defined as AIN1/AIN2,
AIN3/AIN4, and AIN5/AIN6. The available input chan-
nels for each mode are tabulated in Table 5. Note that
DIFF also affects the scanning sequence when the part
is placed in SCAN mode (Table 4).
BOUT:(Default = 0) Burnout Current Bit. Setting BOUT
= 1 connects 100nA current sources to the selected
analog input channel. This mode is used to check that
a transducer has not burned out or opened circuit. The
burnout current source must be turned off (BOUT = 0)
before measurement to ensure best linearity.
RESERVED:(Default = 0) Reserved Bit. A 0 must be
written to this location.
X2CLK:(Default = 0) Times-Two Clock Bit. Setting this
bit to 1 selects a divide-by-2 prescaler in the clock sig-
nal path. This allows use of a higher frequency crystal
or clock source and improves immunity to asymmetric
clock sources.
Table 2. Data Output Rate vs. CLK, Filter Select, and Modulator Frequency Bits* Data rates offering noise-free 16-bit resolution.
Note: When FAST = 0, f-3dB = 0.262 ·Data Rate. When FAST = 1, f-3dB = 0.443 ·Data Rate.
Default condition is in bold print.
Table 3. Special Modes Controlled by M1, M0 (SCAN = 0)
First Bit (MSB)(LSB)
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
Special Function Register (Write-Only)
MDOUT:(Default = 0) Modulator Out Bit. MDOUT = 0
enables data readout on the DOUT pin, the normal con-
dition for the serial interface. MDOUT = 1 changes the
function of the DOUT and INTpins, providing raw, sin-
gle-bit modulator output instead of the normal serial-
data interface output. This allows custom filtering
directly on the modulator output, without going through
the on-chip digital filter. The INTpin provides a clock to
indicate when the modulator data at DOUT should be
sampled (falling edge of INT). Note that in this mode,
the on-chip digital filter continues to operate normally.
When MDOUT is returned to 0, valid data may be
accessed through the normal serial-interface read
operation.
FULLPD:(Default = 0) Complete Power-Down Bit.
FULLPD = 1 forces the part into a complete power-
down condition, which includes the clock oscillator. The
serial interface continues to operate. The part requires a
hardware reset to recover correctly from this condition.
Note:Changing the reserved bits in the special-func-
tion register from the default status of all 0s will select
one of the reserved modes and the part will not operate
as expected. This register is a write-only register.
However, in the event that this register is mistakenly
read, clock 24 bits of data out of the part to restore it to
the normal interface-idle state.
Transfer-Function RegistersThe three transfer-function registers control the method
used to map the input voltage to the output codes. All
of the registers have the same format. The mapping of
control registers to associated channels depends on
the mode of operation and is affected by the state of
M1, M0, DIFF, and SCAN (Tables 8, 9, and 10).
Table 4. SCAN Mode Scanning
Sequences (SCAN = 1)
Table 5. Available Input Channels
(SCAN = 0)
Note:All other combinations reserved.
First Bit (MSB)(LSB)
Special Function Register (Write-Only)
Transfer-Function Register
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
Analog Inputs AIN1 to AIN6Inputs AIN1 and AIN2 map to transfer-function register
1, regardless of scanning mode (SCAN = 1) or single-
ended vs. differential (DIFF) modes. Likewise, AIN3 and
AIN4 inputs always map to transfer-function register 2.
Finally, AIN5 always maps to transfer-function register 3
(input AIN6 is analog common).
CALGAIN and CALOFFWhen not in scan mode (SCAN = 0), A1 and A0 select
which transfer function applies to CALGAIN and
CALOFF. In scan mode (SCAN = 1), CALGAIN and
CALOFF are always mapped to transfer-function regis-
ter 3. Note that when scanning while M1 ≠M0, the scan
sequence includes both CALGAIN and CALOFF chan-
nels (Table 4). CALOFF always precedes CALGAIN,
even though both channels share the same channel ID
tag (Table 11).
Note that changing the status of any
activechannel
control bits will cause INTto immediately transition high
and the modulator/filter to be reset. INTwill reassert
after the appropriate digital-filter settling time. The con-
trol settings of the inactive channels may be changed
freely without affecting the status of INTor causing the
filter/modulator to be reset.
PGA GainBits G2–G0 control the PGA gain according to Table 6.
Unipolar/Bipolar ModeThe U/Bbit places the channel in either bipolar or
unipolar mode. A 0 selects bipolar mode, and a 1
selects unipolar mode. This bit does not affect the ana-
log-signal conditioning. The modulator always accepts
bipolar inputs and produces a bitstream with 50%
ones-density when the selected inputs are at the same
potential. This bit controls the processing of the digital-
filter output, such that the available output bits are
mapped to the correct output range. Note U/Bmust be
set before a conversion is performed; it will not affect
any data already held in the output register.
Selecting bipolar mode does not imply that any input
may be taken below AGND. It simply changes the gain
and offset of the part. All inputs must remain within their
specified operating voltage range.
Offset-Correction DACsBits D3–D0 control the offset-correction DAC. The DAC
range depends on the PGA gain setting and is
expressed as a percentage of the available full-scale
input range (Table 7).
D3 is a sign bit, and D2–D0 represent the DAC magni-
tude. Note that when a DAC value of 0000 is pro-
grammed (the default), the DAC is disconnected from
the modulator inputs. This prevents the DAC from
degrading noise performance when offset correction is
not required.
Transfer-Function Register MappingTables 8, 9, and 10 show the channel-control register
mapping in the various operating modes.
Table 6. PGA Gain Codes
MAX1400
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
Table 8. Transfer-Function Register Mapping—Normal Mode (M1 = 0, M0 = 0)
Table 9. Transfer-Function Register Mapping—Offset-Cal Mode (M1 = 0, M0 = 1)X = Don’t care
X = Don’t care
