TC7652 查看數據表（PDF） - Microchip Technology

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TC7652

Low Noise, Chopper Stabilized Operational Amplifier

Microchip Technology 

TC7652

3.0 DETAILED DESCRIPTION

3.1 Capacitor Connection

Connect the null storage capacitors to the CA and CB

pins with a common connection to the CRET pin (14-pin

TC7652) or to VSS (8-pin TC7652). When connecting to

VSS, avoid injecting load current IR drops into the

capacitive circuitry by making this connection directly

via a separate wire or PC trace.

3.2 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 out-

put pin, just before the device output saturates. This

prevents uncontrolled differential input voltages and

charge build-up on correction storage capacitors. Out-

put swing is reduced.

3.3 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 be tied to VSS. The external clock signal is then

applied to the EXT CLK IN input (Pin 13). An internal

divide-by-two provides 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 external clock can

swing between VDD and VSS, with the logic threshold

about 2.5V below VDD.

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 frequency. This prevents clock intermodula-

tion effects when two TC7652's are used in a differen-

tial amplifier configuration.

FIGURE 3-1:

TEST CIRCUIT

R2

1MΩ

R1

TC7652

1kΩ

-

+

C

R

C

0.1µF

0.1µF

Output

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

external capacitors will slow overload recovery. This

condition 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 within eligible errors

(typical capacitor drift is 10µV/sec).

4.0 TYPICAL APPLICATIONS

4.1 Component Selection

CA and CB (external capacitors)should be in the 0.1µF

to 1µF 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 0.1µF capacitor for slightly lower offset volt-

age. High quality, film type capacitors (polyester or

polypropylene) are recommended, although a lower

grade ceramic may work in some applications. For

quickest settling after initial turn-on, use low dielectric

absorption capacitors (e.g., polypropylene). With

ceramic capacitors, settling to 1µV takes several sec-

onds.

4.2 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.

DS21464B-page 6

© 2002 Microchip Technology Inc.

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