9.2.2 Detailed Design Procedure

9.2.2.1 Setting the Gain

The gain (G) is set by connecting a single external resistor, RG, between pins 1 and 8:

INA128: G = 1 + 50 kΩ / RG

(1)

INA129: G = 1 + 49.4 kΩ / RG

(2)

Commonly used gains and resistor values are shown in Figure 9-1.

The 50-kΩ term in Equation 1 and the 49.4-kΩ term in Equation 2 come from the sum of the two internal

feedback resistors of A1 and A2. These on-chip metal film resistors are laser trimmed to accurate, absolute

values. The accuracy and temperature coefficient of these internal resistors are included in the gain accuracy

and drift specifications in the Electrical Characteristics table.

The stability and temperature drift of the external gain setting resistor, RG, also affects gain. The contribution

of RG to gain accuracy and drift can be directly inferred from Equation 1 and Equation 2. Low resistor values

required for high gain can make wiring resistance important. Sockets add to the wiring resistance, which

contributes additional gain error (possibly an unstable gain error) in gains of approximately 100 or greater.

9.2.2.2 Dynamic Performance

The typical performance curve in Figure 7-1 shows that despite low quiescent current, the INA12x achieve

wide bandwidth even at high gain. This performance is due to the current-feedback topology of the input stage

circuitry. Settling time also remains excellent at high gain.

9.2.2.3 Offset Trimming

The INA12x is laser trimmed for low-offset voltage and low offset voltage drift. Most applications require no

external offset adjustment. Figure 9-2 shows an optional circuit for trimming the output offset voltage. The

voltage applied to the REF pin is summed with the output. The op-amp buffer provides low impedance at the

REF pin to preserve good common-mode rejection.

10kΩ

OPA177

100Ω

100Ω

1/2 REF200

1/2 REF200

V+

R

G

INA128

Ref

V

O

V

IN

V

IN

+

−

±10mV

Adjustment Range

V−

100µA

100µA

Figure 9-2. Optional Trimming of Output Offset Voltage

INA128, INA129

SBOS051F – OCTOBER 1995 – REVISED MAY 2022

www.ti.com

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