3-284

TELCOM SEMICONDUCTOR, INC.

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 amplifier.

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 materials) 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 possible, use low thermoelectric-coeffi-

cient connections. Keep power supply voltages and power

dissipation to a minimum. Use high-impedance loads and

seek maximum separation from surrounding heat-dissipat-

ing elements.

Guarding

To benefit from TC7652 low-input currents, take care

assembling printed circuit boards. Clean boards with alco-

hol or TCE, and blow dry with compressed air. To prevent

contamination, coat boards with epoxy or silicone 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 surrounding 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.

Pin Compatibility

Where possible, the 8-pin device pinout conforms to

such industry standards as the LM101 and LM741. Null-

storing external capacitors connect to pins 1 and 8, which

are usually for offset-null or compensation capacitors. Output

clamp (pin 5) is similarly used. For OP05 and OP07 devices,

replacement of the offset-null potentiometer (connected

compensation capacitor between pins 1 and 8 by two

removal of any connection to pin 5) works for LM101,

µA748, and similar parts.

Because NC pins provide guarding between input and

other pins, the 14-pin device pinout conforms closely to the

LM108. Because this device does not use any extra pins and

does not provide offset-nulling (but requires a compensation

capacitor), some layout changes are necessary to convert to

the TC7652.

CONNECTION OF INPUT GUARDS

INPUT

OUTPUT

R2

R1

Inverting Amplifier

INPUT

OUTPUT

Follower

INPUT

OUTPUT

R2

R1

Noninverting Amplifier

TC7652

TC7652

TC7652

TC7652

LOW NOISE, CHOPPER-STABILIZED

OPERATIONAL AMPLIFIER