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TELCOM SEMICONDUCTOR, INC.

7

6

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Capacitor Connection

avoid injecting load current IR drops into the capacitive

circuitry by making this connection directly via a separate

wire or PC trace.

Output Clamp

In chopper-stabilized amplifiers, the output clamp pin

reduces overload recovery time. When a connection is

made to the inverting input pin (summing junction), a current

path is created between that point and the output pin, just

before the device output saturates. This prevents uncon-

trolled differential input voltages and charge buildup on

correction-storage capacitors. Output swing is reduced.

Clock

The TC7652 has a 550Hz internal oscillator, which is

divided by two before clocking the input chopper switches.

The 275Hz chopping frequency is available at INT CLK OUT

(pin 12) on 14-pin devices. In normal operation, INT/EXT

(pin 14), which has an internal pull-up, can be left open.

An external clock can also be used. To disable the

internal clock and use an external one, the INT/EXT pin must

EXT CLK IN input (pin 13). An internal divide-by-two pro-

vides a 50% switching duty cycle. The capacitors are only

charged when EXT CLK IN is high, so a 50% to 80% positive

duty cycle is recommended for higher clock frequencies.

The output of the internal oscillator, before the divide-

by-two circuit, is available at EXT CLK IN when INT/EXT is

high or unconnected. This output can serve as the clock

input for a second TC7652 (operating in a master/slave

mode), so that both op amps will clock at the same fre-

quency. This prevents clock intermodulation effects when

two TC7652's are used in a differential amplifier configura-

tion.

If the TC7652's output saturates, error voltages on the

external capacitors will slow overload recovery. This condi-

tion can be avoided if a strobe signal is available. The strobe

signal is applied to EXT CLK IN and the overload signal is

applied to the amplifier while the strobe is LOW. In this case,

neither capacitor will be charged. The low leakage of the

capacitor pins allow long measurements to be made with

negligible errors (typical capacitor drift is 10

µV/sec).

APPLICATION NOTES

Component Selection

to 1

µF range. For minimum clock ripple noise, use a 1µF

capacitor in broad bandwidth circuits. For limited bandwidth

applications where clock ripple is filtered out, use a 0.1

µF

capacitor for slightly lower offset voltage. High-quality film-

type capacitors (polyester or polypropylene) are recom-

mended, although a lower grade (ceramic) may work in

some applications. For quickest settling after initial turn-on,

use low dielectric absorption capacitors (e.g., polypropy-

lene).

With ceramic capacitors, settling to 1

µV takes

several seconds.

Static Protection

Although input diodes static-protect all device pins,

avoid strong electrostatic fields and discharges that can

cause degraded diode junction characteristics and produce

increased input-leakage currents.

Latch-Up

Junction-isolated CMOS circuits have a 4-layer (p-n-

p-n) structure similar to an SCR. Sometimes this junction

can be triggered into a low-impedance state and produce

excessive supply current. Therefore, avoid applying voltage

greater than 0.3V beyond the supply rails to any pin. Estab-

lish the amplifier supplies at the same time or before any

input signals are applied. If this is not possible, drive circuits

must limit input current flow to under 1mA to avoid latch-up,

even under fault conditions.

Output Stage/Load Driving

The output circuit is high impedance (about 18k

Ω). With

lesser loads, the chopper amplifier behaves somewhat like

a transconductance amplifier with an open-loop gain propor-

tional to load resistance. (For example, the open-loop gain

is 17dB lower with a 1k

Ω load than with a 10kΩ load.) If the

amp is used only for DC, the DC gain is typically greater than

120dB (even with a 1k

Ω load), and this lower gain is

inconsequential. For wideband, the best frequency response

occurs with a load resistor of at least 10k

Ω. This produces

OUTPUT

R2

R1

0.1 µF

0.1 µF

TC7652

C

R

C

1 k

Ω

1 M

Ω

TEST CIRCUIT

LOW NOISE, CHOPPER-STABILIZED

OPERATIONAL AMPLIFIER

TC7652