LT1167AI datasheet(14/20 Pages) LINER

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LT1167

APPLICATIONS INFORMATION

signal to be processed, set the common mode time

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

differential mode time constant. 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 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 multi-

plied 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 matching op amp like the LT1114, the discrete

solution’s total error is significantly higher than the

LT1167’s total error. The LT1167 has additional advan-

tages over the discrete design, including lower compo-

nent cost and smaller size.

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. This filter 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

V –

V +

IN +

IN –

1167 F05

VOUT

CXCM1

100pF

CXCM2

100pF

CXD

10pF

RS1

1.6k

RS2

1.6k

EXTERNAL RFI

FILTER

LT1167

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

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