Data Sheet

ADA4895-1/ADA4895-2

DC ERRORS

Figure 46 shows a typical connection diagram and the major

dc error sources.

Figure 46. Typical Connection Diagram and DC Error Sources

The ideal transfer function (all error sources set to 0 and infinite

dc gain) can be expressed as follows:

IN

G

F

IP

G

F

OUT

V

R

R

V

R

R

V

×













−

×













+

= 1

(1)

This equation reduces to the familiar forms for noninverting

and inverting op amp gain expressions, as follows:

IP

G

F

OUT

V

R

R

V

×













+

= 1

(2)

IN

G

F

OUT

V

R

R

V

×











 −

=

(3)

The total output voltage error is the sum of the errors due to the

amplifier offset voltage and input currents. The output error due

to the offset voltage can be estimated as follows:

ERROR

OUT

V

=

(4)













+

×













+

−

+

+

G

F

OUT

PNOM

P

CM

OFFSET

R

R

A

V

PSRR

V

V

CMRR

V

V

NOM

1

where:

NOM

OFFSET

V

is the offset voltage at the specified supply voltage,

which is measured with the input and output at midsupply.

CMRR is the common-mode rejection ratio.

PSRR is the power supply rejection ratio.

A is the dc open-loop gain.

The output error due to the input currents can be estimated

as follows:

+

−

×













+

×

−

×













+

×

=

B

G

F

S

B

G

F

G

F

OUT

I

R

R

R

I

R

R

R

R

V

ERROR

1

1

)

||

(

(5)

BIAS CURRENT CANCELLATION

To cancel the output voltage error due to unmatched bias currents

To compensate for the unmatched bias currents at the two inputs,

0

0

Rev. B | Page 17 of 24