PRINCIPLES OF OPERATION
7.5.1 Passive (AMPS V/R) Current Source
For this current source, the voltage source is simply con-
nected in series with the appropriate range resistor as
shown in Figure 7-7. The range resistors used are the same
ones used by the OHMS source. The range resistance
voltage source range used for each current range is listed
in Table 7-3.
In this series circuit configuration, the actual current is
dependent on the voltage burden seen at the output and
is calculated as follows:
From this equation, it can be seen that the smaller the
voltage burden, the closer the output current will be to
the programmed current. For this reason the passive
(AMPS V/R) current source should only be used with
devices (such as a feedback picoammeter) that have a small
(<lOO~V) voltage burden.
To output a programmed current, the microprocessor sets
the voltage source accordingly to compensate for inac-
curacies of the range resistance. For example, if the pro-
grammed current is lO.OOOOpA and the range resistance is
actually lOlGh2, the voltage source wiIl be set to l.OlV (lOpA
x 1OlGR = l.OlV).
r
R RANGE
+
IOUT +
V-SOURCE
:I-
OUTPUT
Figure 7-7. Passive (AMPS V/R Current Source)
7.5.2 Active (AMPS) Current Source
For this current source the voltage source, range resistors
and an electrometer amplifier are configured as shown in
Figure 7-S. A simplified circuit of the active current source
is shown in Figure 7-9. This sourcing method is indepen-
dent of burden voltage. In this configuration, the voltage
source floats on the output of the electrometer amplifier.
As with the passive (AMPS V/R) current source, the
voltage source is set according to the actual value of the
range resistance. The active (AMPS) current source uses
the same range resistors and voltage source ranges as the
passive (AMPS V/R) current source (see Table 7-3).
Table 7-3. Current Range Resistances
-r-
Current
Range
2
PA
20 pA
200 DA
2L4
20 IIA
200 Il.4
2
MA
20 PA
200 pA
2mA
2om.4
L,
(Nominal)
V-Source
Range
1OOGn 2oomv
1OOGhl 2 v
lOGS2 2 v
1Gll
2 v
1OMn
lMn
100 k0
z;
10 kn
2 v
10 kn 20 v
1 kn 20 v
The electrometer amplifier is comprised of U9 and U13.
Operational amplifier U9 is configured as a buffer to pro-
vide the necessary drive current, and U13 provides the
high input impedance and low bias current. The power
supply (*V) for the electrometer amplifier is boot-
strapped to the output voltage of this stage. Thus, the
supply voltage
to
U9 and U13 will always be within 7.2V
of the output (U9, pin 6). This keeps the common mode
voltage at U13s input to zero volts, eliminating bias cur-
rent changes as the burden voltage varies.
The bootstrap is comprised of constant current diodes 45
and Q6 that bias zeners VRl and VR2. The voltage
developed accross VRl and VR2 is buffered by Q3 and 44.
Transistors Q3 and Q4 supply the necessary current to U9
and U10.
Compliance voltage limit is detected by U8 (dual voltage
comparator), VR4, VR5, R14 and R42, and typically occurs
7-8
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