Keithley 610B Instruction Manual page 29

MODELS 610B, 610BR ELECTROMETERS CIRCUIT DESCRIPTION
floating ground keepa the grid et nearly the same potnetial as the cathode,
the current, I, through Q and Rs is constant. I is equal to E/R,, regardless
of the value of G,, as long as the voltage drop across R/ does not exceed the
Multiplier Switch setting.
This circuit provides a true source regardless of
the input.
The Model 610B can then measure the voltage drop across Rx and
indicate the resistance value on its calibrated meter.
Amplifier
I (
+25ovz
I t
Output Gnd
1
I
I
I
1
FIGURE 16. Block Dianram of Model 610B for Normal Method of Measurine Re-
sistance. Circuit designations refer to schematic diagram.
SlO6 is ;he
mltiplier Switch; R,,, is the resistor for a given setting.
G is the un-
known resistance being measured; E is the voltage source (see paragraph 3-5);
R, 1s the range resistor selected with the Range Switch.
Ground references
correspond to the schematic.
b. Guarded Method In the guarded method of resistance measurements (FEED-
BACK Switch inF&ition and the sample resistance connected between the
input terminal, JlOl, and the guard terminal, J104), feedback is applied through
the sample. Refer to Figure 17. The circuit is similar to the normal method,
except for the feedback. This reduces the slowing effect of the instrument's
input capacity.
Leakage error is also reduced since the potential across the
input terminal is small. In this mode, floating ground is connected to the
low impedance side of the input and the output ground is floating. The guard
terminal is at output ground porenfial.
3-6. COULOMBMETER OPERATION. The Model 610B circuit for measuring charge is
similar to that used for an annneter with the fast method. A negative feedback
is applied around a shunt capacitor, Cl03 to C106, selected with the Range
Switch.
The shunt capacitor replaces R, in Figure 15. The stored charge
is proportional to the voltage across the capacitor, which is measured by
the Model 610B voltmeter circuits.
1064R 25