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LT1167CS8

Single Resistor Gain Programmable, Precision Instrumentation Amplifier

Linear Technology 

LT1167

APPLICATIO S I FOR ATIO

To significantly reduce the effect of these out-of-band

signals on the input offset voltage of instrumentation

amplifiers, simple lowpass filters can be used at the

inputs. These filters should be located very close to the

input pins of the circuit. An effective filter configuration is

illustrated in Figure 5, where three capacitors have been

added to the inputs of the LT1167. Capacitors CXCM1 and

CXCM2 form lowpass filters with the external series resis-

tors RS1, 2 to any out-of-band signal appearing on each of

the input traces. Capacitor CXD forms a filter to reduce any

unwanted signal that would appear across the input traces.

An added benefit to using CXD is that the circuit’s AC

common mode rejection is not degraded due to common

mode capacitive imbalance. The differential mode and

common mode time constants associated with the capaci-

tors are:

tDM(LPF) = (2)(RS)(CXD)

tCM(LPF) = (RS1, 2)(CXCM1, 2)

Setting the time constants requires a knowledge of the

frequency, or frequencies of the interference. Once this

frequency is known, the common mode time constants

can be set followed by the differential mode time constant.

To avoid any possibility of inadvertently affecting the

signal to be processed, set the common mode time

constant an order of magnitude (or more) larger than the

differential mode time constant. Set the common mode

RS1 CXCM1

1.6k 0.001µF

IN +

V+

+

CXD

0.1µF

RS2

1.6k

IN –

CXCM2

0.001µF

EXTERNAL RFI

FILTER

RG

LT1167

–

V–

f–3dB ≈ 500Hz

VOUT

1167 F05

Figure 5. Adding a Simple RC Filter at the Inputs to an

Instrumentation Amplifier is Effective in Reducing Rectification

of High Frequency Out-of-Band Signals

time constants such that they do not degrade the LT1167’s

inherent AC CMR. Then the differential mode time con-

stant can be set for the bandwidth required for the appli-

cation. Setting the differential mode time constant close to

the sensor’s BW also minimizes any noise pickup along

the leads. To avoid any possibility of common mode to

differential mode signal conversion, match the common

mode time constants to 1% or better. If the sensor is an

RTD or a resistive strain gauge, then the series resistors

RS1, 2 can be omitted, if the sensor is in proximity to the

instrumentation amplifier.

“Roll Your Own”—Discrete vs Monolithic LT1167

Error Budget Analysis

The LT1167 offers performance superior to that of “roll

your own” three op amp discrete designs. A typical appli-

cation that amplifies and buffers a bridge transducer’s

differential output is shown in Figure 6. The amplifier, with

its gain set to 100, amplifies a differential, full-scale output

voltage of 20mV over the industrial temperature range. To

make the comparison challenging, the low cost version of

the LT1167 will be compared to a discrete instrumentation

amp made with the A grade of one of the best precision

quad op amps, the LT1114A. The LT1167C outperforms

the discrete amplifier that has lower VOS, lower IB and

comparable VOS drift. The error budget comparison in

Table 1 shows how various errors are calculated and how

each error affects the total error budget. The table shows

the greatest differences between the discrete solution and

the LT1167 are input offset voltage and CMRR. Note that

for the discrete solution, the noise voltage specification is

multiplied by √2 which is the RMS sum of the uncorelated

noise of the two input amplifiers. Each of the amplifier

errors is referenced to a full-scale bridge differential

voltage of 20mV. The common mode range of the bridge

is 5V. The LT1114 data sheet provides offset voltage,

offset voltage drift and offset current specifications for the

matched op amp pairs used in the error-budget table. Even

with an excellent matched op amp like the LT1114, the

discrete solution’s total error is significantly higher than

the LT1167’s total error. The LT1167 has additional ad-

vantages over the discrete design, including lower com-

ponent cost and smaller size.

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