Application information

TS4994

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the values of VCC, Rin and Rfeed (AV). To have a good estimation of the VICM value, use the

following formula:

with

The result of the calculation must be in the range:

If the result of the VICM calculation is not in this range, an input coupling capacitor must be

used.

Example: With VCC=2.5V, Rin =Rfeed = 20k and Vic = 2V, we find VICM = 1.63V. This is

higher than 2.5V - 0.9V = 1.6V, so input coupling capacitors are required. Alternatively, you

can change the Vic value.

4.4

Low and high frequency response

In the low frequency region, Cin starts to have an effect. Cin forms, with Rin, a high-pass filter

with a -3dB cut-off frequency. FCL is in Hz.

In the high-frequency region, you can limit the bandwidth by adding a capacitor (Cfeed) in

parallel with Rfeed. It forms a low-pass filter with a -3dB cut-off frequency. FCH is in Hz.

While these bandwidth limitations are in theory attractive, in practice, because of low

performance in terms of capacitor precision (and by consequence in terms of mismatching),

they deteriorate the values of PSRR and CMRR.

The influence of mismatching on PSRR and CMRR performance is discussed in more detail

in the following sections.

Example: A typical application with input coupling and feedback capacitor with FCL =50Hz

and FCH = 8kHz. We assume that the mismatching between Rin1,2 and Cfeed1,2 can be

neglected. If we sweep the frequency from DC to 20kHz we observe the following with

respect to the PSRR value:

●

From DC to 200Hz, the Cin impedance decreases from infinite to a finite value and the

Cfeed impedance is high enough to be neglected. Due to the tolerance of Cin1,2, we

V

ICM

V

CC

R

in

×

2V

ic

R

feed

×

×

+

2R

in

R

feed

+

()

×

--------------------------------------------------------------------------

(V)

=

V

ic

Diff

input+

Diff

input-

+

2

-------------------------------------------------------

(V)

=

0.6V V

ICM

V

CC

0.9V

–

≤≤

)

Hz

(

C

R

2

1

F

in

in

CL

×

×

π

×

=

)

Hz

(

C

R

2

1

F

feed

feed

CH

×

×

π

×

=