LTC1100

7

S

APPLICATI

I FOR ATIO

Common-Mode Rejection

Due to very precise matching of the internal resistors, no

trims are required to obtain a DC CMRR of better than

100dB. However, things change as frequency rises. The

inverting amplifier is in a gain of 1.01 (1.1 for gain of 10),

while the noninverting amplifier is in a gain of 99 (9 for gain

of 10). As frequency rises, the higher gain amplifier hits its

gain-bandwidth limit long before the low gain amplifier,

degrading CMRR. The solution is straightforward — slow

down the inverting amplifier to match the noninverting

amp. Figure 1 shows the recommended circuit. The prob-

lem is less pronounced in the LTC1100CS in gain-of-10

mode; no CMRR trims are necessary.

Figure 2. Overcompensation to Reduce System Bandwidth

100k

10pF

3

LTC1100 • TA02

2

8

6

+

−

+

−

LTC1100

Figure 1. Improving AC CMRR

Overcompensation

Many instrumentation amplifier applications process DC

or low frequency signals only; consequently the 18kHz

(180kHz for G = 10) bandwidth of the LTC1100 can be

reduced to minimize system errors or reduce transmitted

clock noise by using the COMP pin. A feedback cap from

(22.5k for G = 10) to limit the bandwidth, as in Figure 2.

3

LTC1100 • TA03

8

6

+

–

C

7

f

=

3dB

1

2

R

× C

R

= 247k FOR G = 100

22.5k FOR G = 10

INT

B

LTC1100

B

Aliasing

The LTC1100 is a chopper-stabilized instrumentation

amplifier; like all sampled systems it exhibits aliasing

behavior for input frequencies at or near the internal

sampling frequency. The LTC1100 incorporates special-

ized anti-aliasing circuitry which typically attenuates

aliasing products by

≥60dB; however, extremely sensi-

tive systems may still have to take precautions to avoid

aliasing errors. For more information, see the LTC1051/

LTC1053 data sheet.

Single Supply Operation

The LTC1100 will operate on a single 5V supply, and the

common-mode range of the internal op amps includes

ground; single supply operation is limited only by the

output swing of the op amps. The internal inverting

amplifier has a negative saturation limit of 5mV typically,

setting the minimum common-mode limit at 5mV/1.01 (or

1.1 for gain of 10). The inputs can be biased above ground

as shown in Figure 3. Low cost biasing components can be

used since any errors appear as a common-mode term

and are rejected.

The minimum differential input voltage is limited by the

swing of the output op amp. Lightly loaded, it will swing

down to 5mV, allowing differential input voltages as low as

50

µV (450µV for gain of 10). Single supply operation

limits the LTC1100 to positive differential inputs only;

negative inputs will give a saturated zero output.

3

5V

LTC1100 • TA04

8

6

−

+

OUTPUT

0V TO 5V

SENSOR

5

4, 1

5V

1N4148

LTC1100

Figure 3.

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.