Keithley 6512, Instruction Manual

Page: 123 / 156

Maintenance
7-14
X = Relay Energized. * = These relays may also be energized de-
pending on range and function.
7.7.6 Ranging amplifier gain configuration
The ranging amplifier can have one of four gain values: X10,
X1, X0.1, or X0.01. The actual value will depend on the se-
lected range and function, as summarized in Table 7-10. This
information can be used to determine if the ranging amplifier
is working properly. To do so, select the desired range and
function, and apply an appropriate signal to the input of the
electrometer. Measure the signal at the PREAMP and 2V
ANALOG OUTPUT terminals, and see if the correct scaling
factor is applied.
Table 7-10
Ranging amplifier gains
Function Range Gain
Volts 200mV
2V
20V
200V X10
X1
X0.1
X.01
Amps 2pA
20pA
200pA
2nA
20nA
200nA
2µA
20µA
200µA
2mA
20mA X10
X1
X0.1
X10
X1
X0.1
X10
X1
X0.1
X10
X1
Ohms 2k
Ω
20k
Ω
200k
Ω
2M
Ω
20M
Ω
200M
Ω
2G
Ω
20G
Ω
200G
Ω
X10
X1
X1
X1
X1
X1
X1
X0.1
X0.01
Coulombs 200pC
2nC
20nC X10
X1
X.01
External
feedback 200mV
2V
20V X10
X1
X0.1
For example, with the instrument on the 20V range, an input
signal of 19V could be applied. Assuming the input amplifier
is operating properly, the voltage seen at the PREAMP OUT
should also be 19V. The ranging amplifier should apply a
gain of X0.1 in this case, so the voltage seen at the 2V AN-
ALOG OUTPUT should be 1.9V. If the PREAMP OUT sig-
nal is correct, but the 2V ANALOG OUTPUT is not, the
ranging amplifier or associated control circuits are not func-
tioning properly.
7.7.7 A/D converter and display
Use the procedure listed in Table 7-11 to make sure the A/D
converter and display circuits are operating properly. The op-
eration of these circuits should be verified before attempting
to troubleshoot the analog circuitry.
7.7.8 Input and ranging amplifiers
The input and ranging amplifiers condition the input signal,
transforming it into a 0-2V DC voltage that is usable by the
A/D converter. The exact conditioning process will depend
on the selected range and function. With the voltage ranges,
for example, the signal is merely attenuated (20V and 200V
ranges) or amplified (200mV range), and inverted. For the
amps function, the input signal must undergo current-to-
voltage conversion.
The procedures outlined in Tables 7-12 and 7-13 may be
used as an aid in troubleshooting the input amplifier and
ranging circuits. Note that the procedure in Table 7-13 as-
sumes that the input amplifier is operating properly.
WARNING
Up to 300V may be present between
PREAMP OUT and COM.
7.7.9 Digital circuitry
Problems with the digital circuitry could cause erratic oper-
ation. Check the various components associated with the dig-
ital circuitry, including the IEEE- 488 interface, using the
procedure given in Table 7-14.
7.7.10 Display board
Check the display board by using the procedure in Table 7-15.