1B51AN ,Isolated mV/Thermocouple Signal ConditionerSPECIFICATIONS (typical @ +25% and ll, = t15ll unless otherwise noted)
Model IB51AN IBSIBN
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1B51AN
Isolated mV/Thermocouple Signal Conditioner
ANALOG
DEVICES
Isolated mV/I'hermocouple
Signal Conditioner
FEATURES
Functionally Complete Precision Conditioner
High Accuracy
Low Input Offset Tempco: :0.1pV/°C
Low Nonlinearity: :0.025%
High CMR: 160dB (60Hz, G=1000VIV)
High CMV Isolation: 1500V rms Continuous
240V rms Input Protection
Small Package: 1.tt''x2.1''x0,35'' DIP
Isolated Power
Low Pass Filter (fc=3Hz)
Pin Compatible with 1841 Isolated RTD Conditioner
APPLICATIONS
Multichannel Thermocouple Temperature
Measurement
Low Level Data Acquisition Systems
Industrial Measurement & Control Systems
GENERAL DESCRIPTION
The IB51 is a precision, mV/thermocouple signal conditioner
that incorporates a circuit design utilizing transformer based iso-
lation and automated surface mount manufacturing technology.
It provides an unbeatable combination of versatility and perfor-
mance in a compact plastic package. Designed for measurement
and control applications, it is specially suited for harsh environ-
ments with extremely high common-mode interference. Unlike
costlier solutions that require separate dc/dc converters, each
1851 generates its own input side power, providing true, low
cost channel-to-channel isolation.
Functionally, the signal conditioner consists of three basic sec-
tions: chopper stabilized amplifier, isolation and output filter.
The chopper amplifier features a highly stable offset tempco of
:0.lp.V/°C and resistor programmable gains from 2 to 1000.
Wide range zero suppression can be implemented at this stage.
The isolation section has complete input to output galvanic iso-
lation of 1500V rms continuous using transformer coupling tech-
niques. Isolated power of 2mA at t6.2V is provided for ancil-
lary circuits such as zero suppression and open-input detection.
Filtering at 3Hz is implemented by a passive antialiasing filter at
Input Protection:
FUNCTIONAL BLOCK DIAGRAM
SIGNAL
ISOLAYDON
MODULATOR
ll DEMOD
lSOLATION
OSCILLATOR
the front end and a two-pole active filter at the output. Overall
NMR is 60dB and CMR is 160dB min Ci 60Hz, G-- 1000.
The IRS] is specified over -25''C to +85°C and operates over
the industrial (-40''C to +85°C) temperature range.
DESIGN FEATURES AND USER BENEFITS
High Noise Rejection: The combination of a chopper stabilized
front end with a low pass filter provides high system accuracy in
harsh industrial environments as well as excellent rejection of
50/60Hz noise.
The input is internally protected against
continuous application of 240V rms.
Low Cost: The IB51 offers a very low cost per channel for
high performance, isolated, low level signal conditioners.
Wide Range Zero Suppression: This input referred function
is a convenient way to null large input offsets.
Low Pass Filter: The three pole active filter (fc=3Hz) reduces
60Hz noise and aliasing errors.
Small Size: The 1851 package size (1.0"x 2.1"x0.35'') and
functional completeness make it an excellent choice in systems
with limited board space and clearance.
1 MI - SPECIFICATIONS (typical @ +25%: and lls = t15ll unless otherwise noted)
Model lBSlAN IB51BN
. . Rn;
GainEquation G-- I+--" x 2 .
Gain Error 1% max .
Gain Temperature Coefficient' 50ppm/°C *
Gain Nonlinearity :0.035% (:0.05% max) :0.025% (t0.04% max)
OFFSET VOLTAGES
Input Offset Voltage
Initial, Ca) +25°C (Adjustable to Zero)
25wV(100wV max)
vs. Temperature t0.1wVPC (t0.5wVPC max) I
vs. Time, Noncumulative t UW/month max .
Output Offset Voltage
Initial -50mV -25mV
vs. Temperature -175rWPC -50wVPC
INPUT OFFSET CURRENT
Initial 0.6nA (2.5m max) .
vs. Temperature t2.5pAf'C (12.5pA/°C max) '
INPUT BIAS CURRENT
Initial @ +25'C 10nA .
vs. Temperature lOpAPC *
INPUT IMPEDANCE
Power On SOMQ A
Power Off 40kft min A
INPUT VOLTAGE RANGE
Linear Differential Input t 10mV to 25V .
Max CMV, Input to Output
ac, 60Hz, Continuous 1500V rms .
Continuous, dc t2000V *
CMR C4 60Hz, lkn Source Imbalance, G-- 1000 lahiB min .
NMR @ 60Hz 60dB min .
Transient Protection IEEE-STD 472 (SWC) .
INPUT NOISE
Voltage, 0.1Hz to 10Hz, lkn Source Imbalance luV p-p I
RATED OUTPUT
Voltage, 2kn Load, min t 10V *
Current t SmA A
Output Noise, dc to lOOkHz lmV p-p .
Impedance, dc 0.10 *
FREQUENCY RESPONSE
Bandwidth, -3dB dc to 3Hz *
ISOLATED POWER
Voltage, No Load :6.2V:5% .
Current 2mA .
Regulation, No Load to Full Load 7.5% .
Ripple 250mV p-p *
POWER SUPPLY
Voltage, Rated Performance t 15V dc .
Voltage, Operating :13.5V to t18V '
Current, Quiescent +12mA © +15V, -4mh @ -15V .
PSRR 0.1%N I
ENVIRONMENTAL
Tcmpcramre Range
Rated Performance -25°C to +85°C *
Operating -40°C to +85''C .
Storage -40'C to +85°C *
Relative Humidity 0 to 95% @ +60°C *
CASE SIZE 1.0(rx2.l0"x0.35'' *
(25.4x53.3X8.9)mm
'Specifications same as IBSIAN.
1See graph in text.
Specifications subject to change without notice.
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
0.03 035
[076) (as)
ons _L -l MAX
i3.8t) om: I
I |+o.oo(zo.3)rvr
(2.541
o.1so I -
t - on
- - (2.54)
1.00 (25A) MAX m
PIN DESIGNATIONS
PIN DESIGNATION
2 PROT HI
4 ICOM
16 +15V
17 - 15V
23 GND
34 -Veso
35 +VISO
37 GAIN
PROT 2
" Voor
SIGNAL J;
ISOLATION
MODULATOR
GAIN 37 CHOPPER
AMPLIFIER
TIMING
ICOM 4
+V.so 35
- Vero 34
SUPPLY
DEMOOULATOR
POWER TIMING
ISOLATION
" +15V
ll OSCILLATOR " - 15V
23 GND
Functional Block Diagram
INSIDE THE IB51
Referring to the functional block diagram, the t 15V power in-
puts provide power to both the output side circuitry and the
power oscillator. The 25kHz power oscillator provides the tim-
ing information for the signal demodulator and drives power
transformer T2 for the input side power supplies. The second-
ary winding of T2 is half wave rectified and filtered to create the
input side bipolar unregulated supplies.
The signal input (HI) is single-pole filtered for noise rejection
and antialiasing. The protection clamps limit the voltage at
PROT HI to 18V. Thus, a large voltage applied between HI
and input common (1 COM) appears mostly across the input
resistor.
The chopper stabilized gain stage amplifies the differential input
voltage with a gain set by external resistors. The voltage at the
inverting input of the chopper stabilized amplifier (LO) should
be equal to the input voltage at which the desired output voltage
is zero. This is a true input referred zero suppression function.
The signal is amplitude modulated onto a 25kHz carrier and
passed through the signal transformer T1. The synchronous de-
modulator restores the signal to the baseband. A two-pole active
low pass stage filters out clock noise and completes a three-pole
Butterworth filter formed with the input pole.
CHOPPER
AMPLIFIER
A0589 1 25V
Figure 1. Input Gain Setting andZero Supression
USING THE IB51
Gain Setting:
The gain of the 1B51 is controlled on the input side by a pair of
user provided resistors (see Figure l). A feedback resistor of
between 10kf2 and 20kn is required between the feedback pin
(FE) and the gain pin. The gain setting resistor is connected
between the gain pin and input side common (ICOM). The gain
equation is
G - [l + E] X 2
Gains of 2-1000 can be achieved by adjusting this ratio.
The accuracy of the resistor values must be taken into account
when calculating the initial gain accuracy of an application. The
initial accuracy of the 1B51 must then be added to the resistor
errors to predict the total accuracy. Likewise, the ratiometric
temperature coefficient of the gain and feedback resistors must
be added to the temperature coefficient of the IBSI to predict
the total resulting thermal drift.
It is possible to use a trimming potentiometer to correct for ini-
tial gain and system gain errors. The feedback resistor can be
comprised of a resistor in series with a trimming potentiometer,
as long as the total resistance remains between 10kft and 20kf1.
Alternatively, the gain resistor can also be an adjustable resistor.
In general, the greater the trim range, the coarser the resolution.
Zero Suppression:
Since the 11351 is a differential input device, true input referred
zero suppression can be accomplished (see Figure l). A voltage
reference powered by the input side power supplies is applied to
the LO terminal. Since the transfer function is
Vo = (V(HI) - V(LO)) x GAIN
the input voltage for which the desired output is zero should be
applied to the LO pin. The equation is
v, = 1.25(Rz/(R, + Rs))
Any drift of this input zero suppression voltage appears as offset
drift, so a temperature stable reference should be used. The
source impedance at the LO terminal should be kept below
Open Input Detection:
The 1851 can sense an open thermocouple or broken input line
with the addition of an external resistor. By connecting a
220hin resistor between the HI pin and the positive or negative
isolated supply, an open input will cause a positive or negative
full scale output, respectively.
To preserve the normal mode input protection capability of the
IRS], the resistor must be able to withstand 220Vac. A high
voltage rating can be obtained by connecting lower value resis-
tors in series.
Cold Junction Compensation:
When using a thermocouple as an input to the 1B51, a second
thermocouple junction is formed at the terminations of the ther-
mocouple wires, commonly referred to as the cold junction. The
measured output voltage of the sensor is the voltage generated
by the thermocouple minus the voltage generated by the cold
junction.
Since thermocouples are specif1ed with 0V representing 0°C, it
would be ideal to maintain the cold junction at 0°C. A more
practical approach involves adding a temperature dependent
voltage to the thermocouple signal so as to oppose the cold junc-
tion effects. This type of correction is known as cold junction
compensation.
Many different methods are commonly used to implement cold
junction compensation. Usually a thermistor or a semiconductor
sensor is used to generate the cold junction voltage. The slope
of the cold junction voltage must be the same as that of the ther-
mocouple. Therefore, the cold junction compensation depends
on the thermocouple type.
Sometimes, one cold junction compensation sensor is used by a
number of thermocouple channels. This is accomplished by
measuring the temperature of the connection block directly, and
adding the appropriate voltage to each uncompensated thermo-
couple channel after the gain has been taken. In all cases, the
cold junction sensor must be in the thermal proximity with the con-
nection block.
Figure 2 shows a monolithic cold junction compensation device
used with the 1B51. The Analog Devices AC1226 measures the
ambient temperature and generates the appropriate cold junction
voltage for several different thermocouple types.
LO 1851
AC1226''
150kit
''PIN NUMBER DEPENDS ON THERMOCOUPLE TYPE.
SEE AC1226 DATA SHEET FOR DETAILS.
Figure 2. 1851 Cold Junction Compensation
TYPICAL PERFORMANCE CURVES (@Ta--+25%,lls=t15l0
p 1000
'll e,,,-'"""
i m, "s,
-85 0 " " "
TEMPERATURE-t
Gain vs. Temperature
0 I I " "
CAPACITANCE - "
+V,So Ripple vs. Capacitance
160 //"
/ 150 /
, " 100 1000 10k
GAIN _ V/V
CMR vs. Gain
6.2 's
sn "s,
FO 's,,
Z: 's,
D " 1 " 2 "
+V,so vs. Load
