LT1789CS8-10-PBF Ver la hoja de datos (PDF) - Linear Technology

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LT1789CS8-10-PBF

Micropower, Single Supply Rail-to-Rail Output Instrumentation Amplifiers

Linear Technology 

LT1789-1/LT1789-10

APPLICATIONS INFORMATION

Reference Terminal

Input Bias Current Return Path

The output voltage of the LT1789-1/LT1789-10 (Pin 6)

is referenced to the voltage on the reference terminal

(Pin 5). Resistance in series with the REF pin must be

minimized for best common mode rejection. For example,

a 22Ω resistance from the REF pin to ground will not

only increase the gain error by 0.02% but will lower the

CMRR to 80dB.

Output Offset Trimming

The LT1789-1/LT1789-10 is laser trimmed for low offset

voltage so that no external offset trimming is required for

most applications. In the event that the offset needs to be

adjusted, the circuit in Figure 2 is an example of an optional

offset adjust circuit. The op amp buffer provides a low

impedance to the REF pin where resistance must be kept

to a minimum for best CMRR and lowest gain error.

The low input bias current of the LT1789-1/LT1789-10

(19nA) and the high input impedance (1.6GΩ) allow the

use of high impedance sources without introducing sig-

nificant offset voltage errors, even when the full common

mode range is required. However, a path must be provided

for the input bias currents of both inputs when a purely

differential signal is being amplified. Without this path the

inputs will float high and exceed the input common mode

range of the LT1789-1/LT1789-10, resulting in a saturated

input stage. Figure 3 shows three examples of an input bias

current path. The first example is of a purely differential

signal source with a 10kΩ input current path to ground.

Since the impedance of the signal source is low, only one

resistor is needed. Two matching resistors are needed for

higher impedance signal sources as shown in the second

example. Balancing the input impedance improves both

common mode rejection and DC offset. The need for input

resistors is eliminated if a center tap is present as shown

in the third example.

–IN 2 –

OUTPUT

V+

RG

1

8 LT1789-1/-10

6

+IN

3+

REF

5

2

1

LT1880

±10mV

3

ADJUSTMENT RANGE

10mV

100Ω

10k

100Ω

–10mV

V–

1789 F02

Figure 2. Optional Trimming of Output Offset Voltage

–

–

–

THERMOCOUPLE RG

10k

LT1789-1/

LT1789-10

+

MICROPHONE,

HYDROPHONE,

ETC

200k

RG

LT1789-1/

LT1789-10

+

200k

Figure 3. Providing an Input Common Mode Current Path

RG

LT1789-1/

LT1789-10

+

CENTER-TAP PROVIDES

BIAS CURRENT RETURN

1789 F03

1789fc

19

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