© 2002 Microchip Technology Inc.

DS21464B-page 7

TC7652

4.3

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 proportional to load resistance. (For

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

Ω.

load than with a 10k

Ω load.) If the amp is used only for

DC, the DC gain is typically greater than 120dB (even

witha1k

Ω load), and this lower gain is inconsequential.

For wide band, the best frequency response occurs

with a load resistor of at least 10k

Ω. This produces a

6dB/octave response from 0.1Hz to 2MHz, with phase

shifts of less than 2 degrees in the transition region,

where the main amplifier takes over from the null ampli-

fier.

FIGURE 4-1:

CONNECTION OF INPUT GUARDS

4.4

Thermoelectric Effects

The thermoelectric (Seebeck) effects in thermocouple

junctions of dissimilar metals, alloys, silicon, etc. limit

ultra high precision DC amplifiers. Unless all junctions

are at the same temperature, thermoelectric voltages

around 0.1

µV/°C (up to tens of µV/°C for some materi-

als) are generated. To realize the low offset voltages of

the chopper, avoid temperature gradients. Enclose

components to eliminate air movement, especially from

power dissipating elements in the system. Where pos-

sible, use low thermoelectric co-efficient connections.

Keep power supply voltages and power dissipation to a

minimum. Use high impedance loads and seek maxi-

mum separation from surrounding heat disipating ele-

ments.

4.5

Guarding

To benefit from TC7652 low input currents, take care

assembling printed circuit boards. Clean boards with

alcohol or TCE and blow dry with compressed air. To

prevent contamination, coat boards with epoxy or sili-

cone rubber.

Even if boards are cleaned and coated, leakage cur-

rents may occur because input pins are next to pins at

supply potentials. To reduce this leakage, use guarding

to lower the voltage difference between the inputs and

adjacent metal runs. The guard (a conductive ring sur-

rounding inputs) is connected to a low impedance point

at about the same voltage as inputs. The guard

absorbs leakage currents from high voltage pins.

The 14-pin dual-in-line arrangement simplifies guard-

ing. Like the LM108 pin configuration (but unlike the

101A and 741), pins next to inputs are not used.

Input

Output

Inverting Amplifier

Input

Output

Follower

Input

Output

Noninverting Amplifier

TC7652

TC7652

TC7652