9

LT1920

Input and Output Offset Voltage

The offset voltage of the LT1920 has two components: the

output offset and the input offset. The total offset voltage

referred to the input (RTI) is found by dividing the output

offset by the programmed gain (G) and adding it to the

input offset. At high gains the input offset voltage domi-

nates, whereas at low gains the output offset voltage

dominates. The total offset voltage is:

Total input offset voltage (RTI)

= input offset + (output offset/G)

Total output offset voltage (RTO)

= (input offset • G) + output offset

Reference Terminal

The reference terminal is one end of one of the four 10k

resistors around the difference amplifier. The output volt-

age of the LT1920 (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 2

Ω resistance from the REF pin

to ground will not only increase the gain error by 0.02%

but will lower the CMRR to 80dB.

Single Supply Operation

For single supply operation, the REF pin can be at the same

potential as the negative supply (Pin 4) provided the

output of the instrumentation amplifier remains inside the

specified operating range and that one of the inputs is at

least 2.5V above ground. The barometer application on the

front page of this data sheet is an example that satisfies

transducer to ground sets the operating current for the

bridge and also has the effect of raising the input common

mode voltage. The output of the LT1920 is always inside

the specified range since the barometric pressure rarely

goes low enough to cause the output to rail (30.00 inches

of Hg corresponds to 3.000V). For applications that re-

quire the output to swing at or below the REF potential, the

voltage on the REF pin can be level shifted. An op amp is

used to buffer the voltage on the REF pin since a parasitic

series resistance will degrade the CMRR. The application

in the back of this data sheet, Four Digit Pressure Sensor,

is an example.

THEORY OF OPERATIO

Output Offset Trimming

The LT1920 is laser trimmed for low offset voltage so that

no external offset trimming is required for most applica-

tions. 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 minimum for

best CMRR and lowest gain error.

Input Bias Current Return Path

The low input bias current of the LT1920 (2nA) and the

high input impedance (200G

Ω) allow the use of high

impedance sources without introducing additional 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 to either rail and exceed the input common mode

range of the LT1920, 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 10k

Ωinputcurrentpathtoground.Sincethe

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.

2

–IN

OUTPUT

+IN

1

8

10k

100

Ω

100

Ω

–10mV

1920 F02

10mV

5

2

3

1

6

1/2

LT1112

±10mV

ADJUSTMENT RANGE

3

LT1920

REF

Figure 2. Optional Trimming of Output Offset Voltage