13

LT1168

Single Supply Operation

For best results under single supply operation, the REF pin

should be raised above the negative supply (Pin 4) and one

of the inputs should be at least 2.5V above ground. The

barometer application later in this data sheet is an example

the bridge transducer to ground sets the operating current

for the bridge, and with R6, also has the effect of raising the

input common mode voltage. The output of the LT1168 is

always inside the specified range since the barometric

pressure rarely goes low enough to cause the output to clip

(30.00 inches of Hg corresponds to 3.000V). For applica-

tions that require the output to swing at or below the REF

0

–1

6

1168 F02

–4

–6

24

8

–7

–8

–2

–3

–5

10

12

14

G = 2

AREA OF

OPERATION

INPUT COMMON

MODE RANGE IS

BELOW THE CURVE

G = 100

AREA OF

OPERATION

G = 1

AREA OF OPERATION

G = 10

AREA OF

OPERATION

Figure 2. Positive Input Range vs

Output Voltage for Different Gains

–14

1

3

4

5

–4

–2

–6

9

1168 F03

2

–10

–8

–12

0

6

7

8

INPUT COMMON

MODE RANGE IS

ABOVE THE CURVE

G = 2

AREA OF

OPERATION

G = 100

AREA OF

OPERATION

G = 10

AREA OF

OPERATION

G = 1

AREA OF OPERATION

Figure 4. Optional Trimming of Output Offset Voltage

Figure 3. Negative Input Voltage Range

vs Output Voltage for Various Gains

potential, the voltage on the REF pin can be further level

shifted. The application in the front of this data sheet,

Single Supply Pressure Monitor, is an example. An op amp

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

series resistance will degrade the CMRR.

Output Offset Trimming

The LT1168 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 4 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.

2

–IN

OUTPUT

+IN

1

8

10k

100

Ω

100

Ω

–10mV

1168 F04

10mV

5

6

1/2 LT1112

±10mV

ADJUSTMENT RANGE

3

LT1168

REF

Input Bias Current Return Path

The low input bias current of the LT1168 (250pA) 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 LT1168, resulting in a saturated input stage.

Figure 5 shows three examples of an input bias current

THEORY OF OPERATIO