AD7869

–7–

REV. A

500

200

100

50

20

10

50

100

200

1k

2k

10k

20k

100k

FREQUENCY – Hz

REF OUT

OUTPUT WITH

ALL 0s LOADED

REF OUT DECOUPLED

AS SHOWN IN

FIGURE 2

Figure 3. Noise Spectral Density vs. Frequency

INPUT/OUTPUT TRANSFER FUNCTIONS

A bipolar circuit for the AD7869 is shown in Figure 4.

The analog input/output voltage range of the AD7869 is

±3 V.

The designed code transitions for the ADC occur midway be-

tween successive integer LSB values (i.e., 1/2 LSB, 3/2 LSB,

5/2 LSB . . . FS –3/2 LSBs). The input/output code is 2s

Complement Binary with 1 LSB = FS/16384 = 366

µV. The

ideal transfer function is shown in Figure 5.

AD7869*

RO ADC

RI DAC

AGND

*ADDITIONAL PINS OMITTED FOR CLARITY

ANALOG OUTPUT

RANGE = ±3V

ANALOG INPUT

RANGE = ±3V

R1

200

C2

0.1µF

C1

10µF

Figure 4. Basic Bipolar Operation

-FS

2

FS = 6V

1LSB =

FS

16384

0V

011...111

011...110

000...010

000...001

000...000

111...111

111...110

100...001

100...000

INPUT VOLTAGE

OUTPUT

CODE

2

-1LSB

FS

+

Figure 5. Input/Output Transfer Function

OFFSET AND FULL SCALE ADJUSTMENT

In most digital signal processing (DSP) applications, offset and

full-scale errors have little or no effect on system performance.

Offset error can always be eliminated in the analog domain by

ac coupling. Full-scale errors do not cause problems as long as

the input signal is within the full dynamic range of the ADC.

For applications requiring that the input signal range match the

full analog input dynamic range of the ADC, offset and full-

scale errors have to be adjusted to zero.

ADC ADJUSTMENT

Figure 6 has signal conditioning at the input and output of the

AD7869 for trimming the endpoints of the transfer functions of

both the ADC and the DAC. Offset error must be adjusted be-

fore full-scale error. For the ADC, this is achieved by trimming

the offset of A1 while the input voltage, V1, is 1/2 LSB below

ground. The trim procedure is as follows: apply a voltage of

–183

µV (–1/2 LSB) at V1 in Figure 6 and adjust the offset volt-

age of A1 until the ADC output code flickers between 11 1111

1111 1111 (3FFF HEX) and 00 0000 0000 0000 (0000 HEX).

AD7869*

*ADDITIONAL PINS

OMITTED FOR

CLARITY

AGND

A1

V1

INPUT VOLTAGE

RANGE =

±3V

R1

10k

R2

500

R3

10k

R5

10k

R4

10k

A2

R6

10k

R7

500

R8

10k

R10

10k

R9

10k

V0

OUTPUT VOLTAGE

RANGE =

± 3V

Figure 6. AD7869 with Input/Output Adjustment

ADC gain error can be adjusted at either the first code transi-

tion (ADC negative full scale) or the last code transition (ADC

positive full scale). The trim procedures for both cases are as

follows (see Figure 6).

ADC Positive Full-Scale Adjustment

Apply a voltage of 2.99945 V (FS/2 – 3/2 LSBs) at V1. Adjust

R2 until the ADC output code flickers between 01 1111 1111

1110 (1FFE HEX) and 01 1111 1111 1111 (1FFF HEX).

ADC Negative Full-Scale Adjustment

Apply a voltage of –2.99982 V (–FS/2 + 1/2 LSB) at V1 and ad-

just R2 until the ADC output code flickers between 10 0000

0000 0000 (2000 HEX) and 10 0000 0000 0001 (2001 HEX).

DAC ADJUSTMENT

Op amp A2 is included in Figure 6 for the DAC transfer func-

tion adjustment. Again, offset must be adjusted before full scale.

To adjust offset, load the DAC with 00 0000 0000 0000 (0000

HEX) and trim the offset of A2 to 0 V. As with the ADC adjust-

ment, gain error can be adjusted at either the first code transi-

tion (DAC negative full scale) or the last code transition (DAC

positive full scale). The trim procedures for both cases are as

follows:

DAC Positive Full-Scale Adjustment

Load the DAC with 01 1111 1111 1111 (1FFF HEX) and ad-

just R7 until the op amp output voltage is equal to 2.99963 V

(FS/2 – 1 LSB).

DAC Negative Full-Scale Adjustment

Load the DAC with 10 0000 0000 0000 (2000 HEX) and adjust

R7 until the op amp output voltage is equal to –3 V (–FS/2).