16
Pulse “droop” during the hold interval is 170pA/200pF which is
0.85µV/µs; (i.e., 170pA/200pF). In this case, 170pA represents
the typical leakage current of the CA3080A when strobed off. If
C
1
were increased to 2000pF, the “hold-droop” rate will
decrease to 0.085µV/µs, but the slew rate would decrease to
0.25V/µs. The parallel diode network connected between
Terminal 3 of the CA3080A and Terminal 6 of the CA3140
prevents large input signal feedthrough across the input
terminals of the CA3080A to the 200pF storage capacitor when
the CA3080A is strobed off. Figure 24 shows dynamic
characteristic waveforms of this sample-and-hold system.
Current Amplifier
The low input terminal current needed to drive the CA3140
makes it ideal for use in current amplifier applications such
as the one shown in Figure 25 (see Note 14). In this circuit,
low current is supplied at the input potential as the power
supply to load resistor R
L
. This load current is increased by
the multiplication factor R
2
/R
1
, when the load current is
monitored by the power supply meter M. Thus, if the load
current is 100nA, with values shown, the load current
presented to the supply will be 100µA; a much easier current
to measure in many systems.
Note that the input and output voltages are transferred at the
same potential and only the output current is multiplied by
the scale factor.
The dotted components show a method of decoupling the
circuit from the effects of high output load capacitance and
the potential oscillation in this situation. Essentially, the
necessary high frequency feedback is provided by the
capacitor with the dotted series resistor providing load
decoupling.
Full Wave Rectifier
Figure 26 shows a single supply, absolute value, ideal full-
wave rectifier with associated waveforms. During positive
excursions, the input signal is fed through the feedback
network directly to the output. Simultaneously, the positive
excursion of the input signal also drives the output terminal
(No. 6) of the inverting amplifier in a negative going
excursion such that the 1N914 diode effectively disconnects
the amplifier from the signal path. During a negative going
excursion of the input signal, the CA3140 functions as a
normal inverting amplifier with a gain equal to -R
2
/R
1
. When
the equality of the two equations shown in Figure 26 is
satisfied, the full wave output is symmetrical.
NOTE:
14. “Operational Amplifiers Design and Applications”, J. G. Graeme,
McGraw-Hill Book Company, page 308, “Negative Immittance
Converter Circuits”.
Top Trace: Output; 50mV/Div., 200ns/Div.
Bottom Trace: Input; 50mV/Div., 200ns/Div.
Top Trace: Output Signal; 5V/Div, 2µs/Div.
Center Trace: Difference of Input and Output Signals through
Tektronix Amplifier 7A13; 5mV/Div., 2µs/Div.
Bottom Trace: Input Signal; 5V/Div., 2µs/Div.
LARGE SIGNAL RESPONSE AND SETTLING TIME
SAMPLING RESPONSE
Top Trace: Output; 100mV/Div., 500ns/Div.
Bottom Trace: Input; 20V/Div., 500ns/Div.
FIGURE 24. SAMPLE AND HOLD SYSTEM DYNAMIC
CHARACTERISTICS WAVEFORMS
+15V
2
1
100kΩ
0.1µF
-15V
4
5
7
+
CA3140
-
0.1µF
4.3kΩ
10kΩ
6
3
R
1
POWER
SUPPLY
10MΩ
R
2
I
L
R
2
R
1
M
R
L
I
L
x
FIGURE 25. BASIC CURRENT AMPLIFIER FOR LOW CURRENT
MEASUREMENT SYSTEMS
CA3140, CA3140A
