Troubleshooting
2-23
2.11.5 A/D converter tests
Control registers
There are four data words used to configure and control the instrument. The AD_STB and
MUX_STB data words are used to control the A/D converter, multiplexing, and the input gain
configuration. The R1_STB data word is used to control the DCV and ohms configuration, and
the R2_STB data word is used to control the ACV configuration. Each data word is generated in
the digital section and is passed to the four sets of control shift registers.
The four control registers are made up of the following ICs:
AD_STB: U811, U810, U809
MUX_STB: U206, U207
R1_STB: U203, U222, U224
R2_STB: U400, U406, U411, U432
The description of each test provides a summary of bit patterns for each of the registers and iden-
tifies IC terminals.
Test 200.1 — A/D converter communication
Type
Pass/Fail
Fault message
No A/D communication
Description
The instrument is set up in default condition as described in the note above. U222 is set up to
switch common through R259 and S1 pin 4 input to the A/D buffer, U226. U226 is set up for X1
gain through U227 pin 2 to 3 with /X1 pin 1 low. Also, common is switched through R312 and
U242 pins 2 to 3, through R311 and to pin 3 of U253. U253 is the buffer for ADGND. ADGND
is routed to A/D in through the parallel combination of R285 and R286. Common is the input to
A/D_IN. The A/D is triggered until the CB (Charge Balance) counts from triggered readings are
the same value, the value is stored and compared to a zero by design CB value. FS (Final Slope)
counts are also stored. The counts that are used for the limits are derived from input signal levels,
current values to the integrator, integration time, and the clock for U816. The count values are
raw counts and are required for the 200 series tests. Once proper operation of the A/D can be
determined, the software will convert the counts into voltage values.
If the A/D cannot make this measurement, all BIT tests will fail.
Drawing reference
Analog Board; 2002-100
A/D Converter; 2002-160
Components
U816, analog-to-digital conversion circuitry.
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