®

INA111

8

The INA111 exhibits approximately 6dB rise in gain at

2MHz in unity gain. This is a result of its current-feedback

topology and is not an indication of instability. Unlike an op

amp with poor phase margin, the rise in response is a

predictable +6dB/octave due to a response zero. A simple

pole at 700kHz or lower will produce a flat passband

response (see Input Filtering).

The INA111 provides excellent rejection of high frequency

common-mode signals. The typical performance curve,

“Common-Mode Rejection vs Frequency” shows this be-

havior. If the inputs are not properly balanced, however,

common-mode signals can be converted to differential sig-

other, from the source signal all the way to the input pins. If

possible use a ground plane under both input traces. Avoid

running other potentially noisy lines near the inputs.

NOISE AND ACCURACY PERFORMANCE

The INA111’s FET input circuitry provides low input bias

current and high speed. It achieves lower noise and higher

accuracy with high impedance sources. With source imped-

ances of 2k

Ω to 50kΩ the INA114 may provide lower offset

voltage and drift. For very low source impedance (

≤1kΩ),

the INA103 may provide improved accuracy and lower

noise.

OFFSET TRIMMING

The INA111 is laser trimmed for low offset voltage and

drift. Most applications require no external offset adjust-

ment. Figure 2 shows an optional circuit for trimming the

output offset voltage. The voltage applied to Ref terminal is

summed at the output. Low impedance must be maintained

at this node to assure good common-mode rejection. The op

amp shown maintains low output impedance at high fre-

quency. Trim circuits with higher source impedance should

be buffered with an op amp follower circuit to assure low

impedance on the Ref pin.

INPUT BIAS CURRENT RETURN PATH

The input impedance of the INA111 is extremely high—

Ω. However, a path must be provided for

the input bias current of both inputs. This input bias current

is typically less than 10pA. High input impedance means

that this input bias current changes very little with varying

input voltage.

Input circuitry must provide a path for this input bias current

if the INA111 is to operate properly. Figure 3 shows various

provisions for an input bias current path. Without a bias

current return path, the inputs will float to a potential which

exceeds the common-mode range of the INA111 and the

input amplifiers will saturate.

If the differential source resistance is low, the bias current

return path can be connected to one input (see the thermo-

couple example in Figure 3). With higher source impedance,

using two resistors provides a balanced input with possible

advantages of lower input offset voltage due to bias current

and better high-frequency common-mode rejection.

INA111

1M

Ω

1M

Ω

INA111

10k

Ω

Thermocouple

INA111

Center-tap provides

bias current return.

Crystal or

Ceramic

Transducer

FIGURE 3. Providing an Input Common-Mode Current Path.

FIGURE 2. Optional Trimming of Output Offset Voltage.

INPUT COMMON-MODE RANGE

The linear common-mode range of the input op amps of the

INA111 is approximately

±12V (or 3V from the power

supplies). As the output voltage increases, however, the

linear input range will be limited by the output voltage swing

is related to the output voltage of the complete amplifier—

see performance curve “Input Common-Mode Range vs

Output Voltage”.

INA111

V

IN

V

IN

R

G

–

+

10k

V

O

OPA177

Ref

±10mV

Adjustment Range

100

100

100µA

1/2 REF200

100µA

1/2 REF200

V+

V–

NOTE: (1) For wider trim range required

in high gains, scale resistor values larger

+

–