ADV7402KST-110 ,Integrated Multi-Format SDTV / HDTV Video Decoder and RGB Graphics DigitizerANALOG
DEVICES
AN-354
APPLICATION NOTE
ONE TECHNOLOGY WAY 0 PD. BOX 9106 o NORWOOD, MASSACH ..
ADV7403BSTZ-110 , 12-Bit, Integrated, Multiformat SDTV/HDTV Video Decoder and RGB Graphics Digitizer
ADV7441ABSTZ-170 , 10-Bit Integrated, Multiformat SDTV/HDTV Video Decoder, RGB Graphics Digitizer, and 2:1 Multiplexed HDMI/DVI Interface
ADV7441ABSTZ-170 , 10-Bit Integrated, Multiformat SDTV/HDTV Video Decoder, RGB Graphics Digitizer, and 2:1 Multiplexed HDMI/DVI Interface
ADV7511-KSTZ , 225 MHz, High Performance HDMI Transmitter with ARC
ADV7520NKBBCZ-80 , Low Power HDMI/DVI Transmitter with Consumer Electronic Control (CEC)
AM27S23APC , 2048-BIT (256X8) BIPOLAR PROM
AM27S23PC , 2048-BIT (256X8) BIPOLAR PROM
AM27S31 , (512X8) Bipolar PROM
AM2833DC , 1024-Bit Static Shift Registers
AM2841ADC ,64 x 4 BITS FIRST-IN FIRST-OUT MEMORIESAm3341/ 2841/ 2841A
Distinctive Characteristics
. "Plug- In" replacement for Fairchild 3341
..
AM2841DC ,64 x 4 BITS FIRST-IN FIRST-OUT MEMORIESAm3341/ 2841/ 2841A
Distinctive Characteristics
. "Plug- In" replacement for Fairchild 3341
..
AD5212BD-AD5212TD/883B-ADV7402KST-110
12-Bit Succesive Approximation High Accuracy A/D Converters
ANALOG
DEVICES
AN-354
APPLICATION NOTE
ONE TECHNOLOGY WAY It PD. BOX 9106 o NORWOOD, MASSACHUSETTS 02062-9106 It 617/329-4700
Ask the Applications Engineer - 1
Multi Troubles
by James Bryant
MULTI TROUBLES
My multiplexed ADC system is misbehaving . . .
Before you go any further, have you grounded all unused
multiplexer channels
No. But how did you know
Because the floating terminal is one of the commonest causes
of problems in systems containing CMOS multiplexers. Un-
used MUX inputs and outputs (whether integrated into a
multiplexed ADC or part of a self-contained MUX chip) can
pick up signals from stray fields and inject them into the
device's substrate, turning on spurious substrate devices.
Then, even when the unused channel is turned off, the per-
formance of the on-channel may be badly degraded (at the
unlikely extreme, the injection may turn on a spurious four-
layer device and destroy some chips).
Whenever a MUX is used, all its inputs and outputs must be
connected to a potential between its supply rails. The best way
to deal with unused channels is to ground them, but they may
be connected to a more-convenient potential within the rails.
TROUBLE FROM THE START
Q: To save power, my ADC is powered up only to make a measure-
ment. The system is very accurate in continuous operation, but
unpredictable when power is strobed. Why
When an ADC's power is switched on only to perform a
conversion, it may misbehave for three reasons: slow reference
turn-on, random initial logic states, and system latch-up.
For various reasons-thermal stabilization, capacitance charg-
ing, slow starting of regenerative current mirrors using PNP
transistors in band-gap references--- it is not uncommon for
some voltage references to have relatively large errors for
many milliseconds after power-up. Such errors in an ADC's
external or internal reference during conversion lead to inac-
curate results.
*Much useful information about logarithmic and other analog function circuits
can be found in the Nonlinear C ircuits Handbook, published by Analog Devices
($5.95), P.O.Box 9902, Norwood MA 02062.
Nee. also Analog Dialogue 19-l (1985), pp, 3-6.
Reprinted from Analog Dialogue 22-2 1988
At turn-on, a typical ADC's logic will be-in a random state; for
a conversion triggered at that time, the ADC may not be able
to perform correctly. With one conversion triggered, the logic
should return to its correct pre-conversion state-but cases
exist where two conversion cycles are necessary before the
ADC is certain to perform a valid conversion. Hence, a good
general rule is to perform two "dummy" conversions after
powerup before relying on the results. (It is also well to recall
that some ADCs react badly to having a conversion triggered
before the previous conversion is complete; when this hap-
pens, one or two "dummy"' conversions may be needed to
return the logic to a known state.)
If an ADC's external logic is arranged so that the end of the
ADC "Busy" signal starts a delay which ends with the start of
the next conversion, it is important to realize that if the
converter powers up in the Busy state, the Busy signal may
remain latched up until a conversion Start pulse has been
received. In this case, such a system cannot self-start. If the
Busy signal is always present on power-up the problem is
almost certain to be recognized-and addressed-during the
design of the system; but if the Busy signal is only occasionally
present on power-up the system may latch unpredictably. As a
rule, control signals to an ADC during start-up should not
depend on the logical state of Busy.
ABOUT LOG COMPENSATION RESISTORS
Q: Designs of logarithmic circuits, including those using the AD538
YfZIXr' unit: (For example, Figure 6 from the AD538 Multi-
function Unit data sheetf) call for "kT/q compensation resistors. "
What are they and where do I get them
: The VBE difference across two opposed silicon junctions, one
carrying a current, I , and the other a current, leer, is (kT/q)
In (I/IREF). Here, k/q is the ratio of Boltzmann's constant to
the charge on an electron (about 1/ 11,605 10V), and T is the
absolute temperature in kelvins.
. Although employing similar junctions in isothermal pairs
eliminates the effects of temperature-sensitivity of reverse sat- 1
' uration current, the kT/q term is still temperature-dependent. . A0589 3:23:
To eliminate this dependency in the application, the logarith- -t.2V
mic voltage must be used in a circuit whose gain is inversely 'sral' mm 1, A “m
proportional to the absolute temperature of the junctions. v, 1'i'i',i:',sr'ir'-'1')'u''vTg',', m D "
Over a reasonable range of temperatures near 20°C, this INPUT ---E 1 b0G4tATN2 "
may be arranged by the use of a gain-setting 1-kn resistor r2Y 7!; am 25. :El tg',xu,
having a positive temperature coefficient of approximately T . . REF P." 'ESF---'-''''
3,400ppm/oC--and keeping it at the same temperature as the {i +ZV s'Giitt)is_ii.,hy
junctions. 3,400p;°r:7"é{ Cf 6 w, ' A0538 'au-c-EY,
A 3,500 ppm/°C resistor is available from Tel Laboratories, Voortr-- +3500 '""'"H E: 2: t, EV
154G Harvey Road, Londonderry, New Hampshire 03053 f'-", '-"I mm" ask v,
(603)-625-8994, Telex: (710)-220-1844, designated Q-81, and t2Ti,,,, u EF--
from the Precision Resistor Co. Inc., 10601, 75th. St., Largo, 23%; It', V =(1VILOG y,
Florida 33543 [(813)-541-5771 Telex: 821788], as the PT146. T Jh% 75 our ”(v)
VOSADJUST
Analog Devices offices in most European countries are aware
of local suppliers of these resistors.
