3-276

TELCOM SEMICONDUCTOR, INC.

Clock Operation

The internal oscillator is set for a 200Hz nominal chop-

ping frequency on both the 8- and 14-pin DIPs. With the

14-pin DIP TC7650, the 200Hz internal chopping frequency

is available at the internal clock output (pin 12). A 400Hz

nominal signal will be present at the external clock input pin

(pin 13) with INT/EXT high or open. This is the internal clock

signal before a divide-by-two operation.

Figure 1. TC7650 Contains a Nulling and Main Amplifier. Offset Correction Voltages Are Stored on Two External Capacitors.

The 14-pin DIP device can be driven by an external

clock. The INT/EXT input (pin 14) has an internal pull-up and

may be left open for internal clock operation. If an external

the internal clock. The external clock signal is applied to the

external clock input (pin 13).

The external clock amplitude should swing between

At low frequencies the external clock duty cycle is not

critical, since an internal divide-by-two gives the desired

50% switching duty cycle. The offset storage correction

capacitors are charged only when the external clock input is

high. A 50% to 80% external clock positive duty cycle is

desired for frequencies above 500Hz to guarantee tran-

sients settle before the internal switches open.

The external clock input can also be used as a strobe

input. If a strobe signal is connected at the external clock

input so that it is LOW during the time an overload signal is

applied, neither capacitor will be charged. The leakage

currents at the capacitors pins are very low. At 25

°C a typical

TC7650 will drift less than 10

µV/sec.

Output Clamp

Chopper-stabilized systems can show long recovery

times from overloads. If the output is driven to either supply

rail, output saturation occurs. The inputs are no longer held

tial signal as an offset and tries to correct it by charging the

external capacitors. The nulling circuit also saturates. Once

the input signal returns to normal, the response time is

lengthened by the long recovery time of the nulling amplifier

and external capacitors.

Through an external clamp connection, the TC7650

Figure 2. Nulling Capacitor Connection

Nulling Capacitor Connection

The offset voltage correction capacitors are connected

tion should be made through a separate PC trace or wire to

avoid voltage drops. The capacitors outside foil, if possible,

NULL

MAIN

AMPLIFIER

NULL

AMPLIFIER

GAIN = A

B

A

B

A

–

+

CB

CA

TC7650

NULL

–

+

V

–

V

+

GAIN = A , OFFSET = V

N

OSN

VOUT

ANALOG INPUT

M

VDD

VSS

6

4

8

1

3

2

7

CA

CB

+

–

VDD

TC7650

10

1

8

2

5

4

11

CA CB

+

–

7

VSS

14-PIN PACKAGE

8-PIN PACKAGE

TC7650

TC7650

CHOPPER-STABILIZED

OPERATIONAL AMPLIFIER