HP 3457A - 8-81. Autocal (ACAL); AC Amplifier DC Offset Auto Calibration; AC Amplifier Attenuator Flatness Auto Calibration

1. The A/D Controller RAM Test writes 55, 00 to each RAM location and reads it back. A RAM test
failure will force the reported ROM checksum result to be in error. On failure, the AUXERR? com-
mand will return the value 2.
2. The A/D Controller ROM Test performs a checksum on the ROM and transmits the results to Main
Controller ALU601. On failure, the AUXERR? command will return the value 2.
A subset of self test | is diagnostic | which will cause the A/D Controller to continually loop through
the Test | routine, Executing DIAGNOSTIC 1 allows monitoring of signals while troubleshooting.
Test 2 is a cross-isolation data communications test that transmits data between the Main Controller
(AI1U601) and the A/D Controller (A2U501) through opto-isolators A2U509/U510. The Main
Controller will transmit a sequence of codes to the A/D Controller which will "ECHO BACK" these
transmissions, as it receives them, to the Main Controller. The data returned to the Main Controller is
then checked for accuracy. If the Main Controller finds an error in the returned data, the error an-
nunciator will flash and executing the AUXERR? command will return a value of 4.
During this procedure, the Main Controller will wait for a response from the A/D Controller. If the
Main Controller never receives a response from the A/D Controller then it will fall into diagnostic 2
and the display will indicate "FAILED".
The data is transmitted in [1 bit frames consisting of one start bit, 8 data bits, one parity bit and one
stop bit. The data is decremented from decimal 16 to decimal |, with each of the patterns being in-
dependently checked for errors. The A/D Controller remains in the "ECHO BACK" mode until it
receives a decimal 0 data byte. At that time it will execute its ’soft reset’. routine. This will set the A/D
Controller to its operational state where it will wait for the Main Controller to direct further activity,
A subset of self test 2 is diagnostic 2 which continuously sends only 4 distinct bytes, FF, OF, 55 and 01
between the A/D Controller and the Main Controller for troubleshooting purposes.
Refer to Section 8-77. Self Test (Test) for information about tests 4 - 7.
8-81. Autocal (ACAL)
The Autocal routine is used to calibrate the AC amplifier DC offset and the Attenuator Flatness on AC
assembly A3, and to calibrate the ohms precharge amplifier and check the extended ohms range (300
Mohm - 3 Gohm) on assembly A2. Each part of this routine is discussed below,
AC Amplifier DC Offset Auto Calibration
The DC Offset voltage of the amplifier circuitry on assembly A3 is calibrated on the 300V range and
3V range to correct for the different DC errors resulting from the two system gains of x10 and x100.
System gain is defined as the gain from A3Q307 pin 3 to A3U304 pin 6 and is determined by the switch
positions in A3U303. During Autocal of the DC Offset. the AC amplifier input is grounded by FET
switch A3Q304 and any offset voltage from amplifier A3Q307/U302 is coupled through analog switch
A3U303 to amplifier A3U304 which amplifies the error and adds any it may have. This offset error is
measured and the results are used by the A/D Controller to set DAC A2U217 which provides an offset
correction signal to the AC amplifier. Figure 8-12 illustrates some of the circuitry used during DC
Offset Autocal.
AC Amplifier Attenuator Flatness Auto Calibration
Two input attenuators on assembly A3 are calibrated during an Autocal. One input attenuator is used
for the 30mV and 300mV ranges and the other is used for the 3V, 30V and 300V ranges. During
HP 3457A Multimeter
8-46