CA3140AM datasheet(18/24 Pages) RENESAS

18 / 24 page

CA3140, CA3140A

FN957 Rev.10.00

Page 18 of 24

Jul 11, 2005

Pulse “droop” during the hold interval is 170pA/200pF which is

0.85

V/s; (i.e., 170pA/200pF). In this case, 170pA represents

the typical leakage current of the CA3080A when strobed off. If C1

were increased to 2000pF, the “hold-droop” rate will decrease to

0.085

V/s, but the slew rate would decrease to 0.25V/s. 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 RL. This load current is increased by the

multiplication factor R2/R1, 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 100

A; 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 -R2/R1. 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, 2

s/Div.

Center Trace: Difference of Input and Output Signals through

Tektronix Amplifier 7A13; 5mV/Div., 2

s/Div.

Bottom Trace: Input Signal; 5V/Div., 2

s/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

100k



0.1

F

-15V

CA3140

0.1

F

4.3k



10k



POWER

SUPPLY

10M



FIGURE 25. BASIC CURRENT AMPLIFIER FOR LOW CURRENT

MEASUREMENT SYSTEMS

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