3-275

TELCOM SEMICONDUCTOR, INC.

7

6

5

4

3

1

2

8

specified.

Symbol

Parameter

Test Conditions

Min

Typ

Max

Units

Supply

Operating Supply Range

4.5

—

16

V

Supply Current

No Load

—

2

3.5

mA

PSRR

Power Supply

120

130

dB

Rejection Ratio

NOTES: 1. See "Output Clamp" discussion.

2. Output clamp not connected. See typical characteristics curves for output swing versus clamp current characteristics.

3. Limiting input current to 100

µA is recommended to avoid latch-up problems.

CHOPPER-STABILIZED

OPERATIONAL AMPLIFIER

TC7650

Theory of Operation

Figure 1 shows the major elements of the TC7650.

There are two amplifiers (the main amplifier and the nulling

amplifier), and both have offset-null capability. The main

amplifier is connected full-time from the input to the output.

The nulling amplifier, under the control of the chopping

frequency oscillator and clock circuit, alternately nulls itself

and the main amplifier. Two external capacitors provide the

required storage of the nulling potentials and the necessary

nulling-loop time constants. The nulling arrangement oper-

ates over the full common-mode and power-supply ranges,

and is also independent of the output level, thus giving

Careful balancing of the input switches minimizes chop-

per frequency charge injection at the input terminals, and the

feed-forward-type injection into the compensation capacitor

that can cause output spikes in this type of circuit.

The circuit's offset voltage compensation is easily shown.

With the nulling inputs shorted, a voltage almost identical to

effective offset voltage at the null amplifier input is:

(1)

After the nulling amplifier is zeroed, the main amplifier is

zeroed; the A switches open and B switches close.

The output voltage equation is:

(3)

As desired, the device offset voltages are reduced by

the high open-loop gain of the nulling amplifier.

Output Stage/Loading

The output circuit is a high-impedance stage (approxi-

mately 18k

Ω). With loads less than this, the chopper ampli-

fier behaves in some ways like a transconductance amplifier

whose open-loop gain is proportional to load resistance. For

example, the open-loop gain will be 17dB lower with a 1k

Ω

load than with a 10k

Ω load. If the amplifier is used strictly for

DC, the lower gain is of little consequence, since the DC gain

is typically greater than 120dB, even with a 1k

Ω load. In

wideband applications, the best frequency response will be

achieved with a load resistor of 10k

Ω or higher. This results

in a smooth 6 dB/octave response from 0.1Hz to 2 MHz, with

phase shifts of less than 10

° in the transition region, where

the main amplifier takes over from the null amplifier. The

clock frequency sets the transition region.

Intermodulation

Previous chopper-stabilized amplifiers have suffered

from intermodulation effects between the chopper frequency

and input signals. These arise because the finite AC gain of

the amplifier results in a small AC signal at the input. This is

seen by the zeroing circuit as an error signal, which is

chopped and fed back, thus injecting sum and difference

frequencies, and causing disturbances to the gain and

phase versus frequency characteristics near the chopping

frequency. These effects are substantially reduced in the

TC7650 by feeding the nulling circuit with a dynamic current

corresponding to the compensation capacitor current in

such a way as to cancel that portion of the input signal due

to a finite AC gain. The intermodulation and gain/phase

disturbances are held to very low values, and can generally

be ignored.

1

Substituting (1)

]

[