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LTC2420

TYPICAL APPLICATIO S

Single-Chip Instrumentation Amplifier

for the LTC2420

The circuit in Figure 37 is a simple solution for processing

differential signals in pressure transducer, weigh scale or

strain gauge applications that can operate on a supply

voltage range of

±5V to ±15V. The circuit uses an LT®1920

single-chip instrumentation amplifier to perform a differ-

ential to single-ended conversion. The amplifier’s output

voltage is applied to the LTC2420’s input and converted to

a digital value with an overall accuracy exceeding 17 bits

(0.0008%). Key circuit performance results are shown in

Table 5.

The practical gain range for this topology as shown is from

5 to 100 because the LTC2420’s wide dynamic range

makes gains below 5 virtually unnecessary, whereas gains

up to 100 significantly reduce the input referred noise.

The optional passive RC lowpass filter between the

amplifier’s output and the LTC2420’s input attenuates

high frequency noise and its effects. Typically, the filter

reduces the magnitude of averaged noise by 30% and

improves resolution by 0.5 bit without compromising

linearity. Resistor R2 performs two functions: it isolates

C1 from the LTC2420’s input and limits the LTC2420’s

input current should its input voltage drop below –300mV

or swing above VCC + 300mV.

The LT1920 is the choice for applications where low cost

is important. For applications where more precision is

required, the LT1167 is a pin-to-pin alternative choice with

a lower offset voltage, lower input bias current and higher

gain accuracy than the LT1920. The LT1920’s maximum

total input-referred offset (VOST) is 135µV for a gain of

100. At the same gain, the LT1167’s VOST is 63µV. At gains

of 10 or 100, the LT1920’s maximum gain error is 0.3%

and its maximum gain nonlinearity is 30ppm. At the same

gains, the LT1167’s maximum gain error is 0.1% and its

maximum gain nonlinearity is 15ppm. Table 6 summa-

rizes the performance of Figure 37’s circuit using the

LT1167.

Figure 37. The LT1920 is a Simple Solution That Converts a Differential Input

to a Ground Referred Single-Ended Signal for the LTC2420

VS+

VS–

VIN

–

RG**

DIFFERENTIAL

INPUT

††

*OPTIONAL—SEE TEXT

**RG = 49.4k/(AV – 1): USE 5.49k FOR AV = 10; 499Ω FOR AV = 100

†USE SHORT LEAD LENGTHS

†

2420 F37

CHIP SELECT

SERIAL DATA OUT

SERIAL CLOCK

SDO

SCK

0.1

µF

0.1

µF

R1*

Ω

C1*

µF

SINGLE POINT

“STAR” GROUND

R2*

10k

0.1

µF

VIN

VREF

VREFIN

VCC

GND

LTC2420

LT1920

LTC2420 Datasheet(PDF) 31 Page - Linear Technology

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