ADA4896-2ACPZ-R7 datasheet(18/28 Pages) AD

18 / 28 page

ADA4896-2/ADA4897-1/ADA4897-2

Data Sheet

Rev.

| Page 18 of 28

DC ERRORS

Figure 46 shows a typical connection diagram and the major

dc error sources.

– VIN +

– VIP +

IB+

IB–

+ VOUT –

+ VOS –

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 written as

OUT

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

= 1

(1)

This equation reduces to the familiar forms for noninverting

and inverting op amp gain expressions, as follows:

For noninverting gain (VIN = 0 V)

OUT

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

= 1

(2)

For inverting gain (VIP = 0 V)

OUT

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛ −

(3)

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

amplifier offset voltage and input currents. The output error

due to the offset voltage can be estimated as

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⎟

⎠

⎞

⎜

⎝

⎛

−

OUT

PNOM

OFFSET

OUT

PSRR

CMRR

NOM

ERROR

(4)

where:

is the offset voltage at the specified supply voltage,

which is measured with the input and output at midsupply.

VCM is the common-mode voltage.

VP is the power supply voltage.

VPNOM is the specified power supply voltage.

CMRR is the common-mode rejection ratio.

PSRR is the power supply rejection ratio.

A is the dc open-loop gain.

NOM

OFFSET

The output error due to the input currents can be estimated as

−

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

OUT

ERROR

)

(

(5)

BIAS CURRENT CANCELLATION

To cancel the output voltage error due to unmatched bias

currents at the inputs, RBP and RBN can be used (see Figure 47).

RBP

RBN

Figure 47. Using RBP and RBN to Cancel Bias Current Error

To compensate for the unmatched bias currents at the two

inputs, set RBP and RBN as shown in Table 11.

Table 11. Setting RBN and RBP to Cancel Bias Current Errors

Value of RF||RG

Value of RBP (Ω)

Value of RBN (Ω)

Greater Than RS

RF||RG − RS

Less Than RS

RS − RF||RG

Table 12 shows sample values for RBP and RBN when RF||RG > RS

and when RF||RG < RS.

Table 12. Examples of RBN and RBP Settings

Gain

RF (Ω)

RG (Ω)

RS (Ω)

RBP (Ω)

RBN (Ω)

249

74.5

+10

249

27.4

25.3

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