AD7669AR datasheet(16/20 Pages) AD

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REV. B

AD7569/AD7669

APPLYING THE AD7569/AD7669 ADC

The analog input on the AD7569/AD7669 accepts the same

four input ranges as the output ranges on the DAC. Whatever

output range is selected for the DAC also applies to the input

range of the ADC.

Although separate AGNDs exist for both the DAC and ADC to

minimize crosstalk, writing data to the DAC while the ADC is

performing a conversion may result in an incorrect conversion

from the ADC due to an interaction of currents between the

DAC and ADC. Therefore, to ensure correct operation of the

ADC, the DAC register should not be updated while the ADC

is converting.

UNIPOLAR OPERATION

The circuit of Figure 21 shows the AD7569 configured for both

an input and output range of 0 V to +1.25 V (the AD7669 con-

figuration is similar). The nominal transfer characteristic for this

range is shown in Figure 22. The output code is Natural Binary

with 1 LSB = (1.25/256)V = 4.88 mV.

As before, to achieve the unipolar 0 V to +2.5 V input range,

VSS is connected to 0 V, and the RANGE input is tied to a logic

high. The nominal transfer characteristic is as in Figure 22 but,

in this case, 1 LSB = (2.5/256)V = 9.76 mV.

Figure 22. Nominal Transfer Characteristic for Unipolar

(0 V to +1.25 V) Operation

BIPOLAR OPERATION

The analog input of the AD7569/AD7669 ADC is configured

for bipolar inputs when VSS = –5 V. The output code provided

by the part is twos complement. Figure 23 shows the transfer

function for bipolar (–1.25 V to +1.25 V) operation. The LSB

size for this range is (2.5/256)V = 9.76 mV.

The transfer function for the –2.5 V to +2.5 V range is identical

to that of Figure 23, but now FS = 5 V and the LSB size is

(5/256)V = 19.5 mV.

ADC OFFSET AND FULL-SCALE ERROR ADJUSTMENT

In most Digital Signal Processing (DSP) applications, offset and

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

Figure 23. Nominal Transfer Characteristic for Bipolar

(–1.25 V to +1.25 V) Operation

typical example is a digital filter where an ac analog signal is

quantized by the ADC, digitally processed and recreated using

the DAC. In these types of applications, the offset error can be

eliminated by ac coupling the recreated signal. Full-scale error

effect is linear and does not cause problems as long as the input

signal is within the full dynamic range of the ADC. An impor-

tant parameter in DSP applications is Differential Nonlinearity,

and this is not affected by either offset or full-scale error.

In applications where absolute accuracy is important ADC off-

set and full-scale error can be adjusted to zero. Figure 24 shows

the additional components required for offset and full-scale er-

ror adjustment. Offset error must be adjusted before full-scale

error. Zero offset is achieved by adjusting the offset of the op

amp driving VIN (i.e., A1 in Figure 23). In unipolar applica-

tions, for zero offset error, apply 1/2 LSB at the analog input

and adjust the op amp offset voltage until the ADC output code

flickers between 0000 0000 and 0000 0001. For zero full-scale

error, apply an analog input of FS – 3/2 LSBs and adjust R1 un-

til the ADC output code flickers between 1111 1110 and 1111

1111.

In bipolar applications, to adjust for bipolar zero offset, apply

–1/2 LSB at the analog input and adjust the op amp offset volt-

age until the output code flickers between 1111 1111 and 0000

0000. For zero full-scale error, apply +FS/2 – 3/2 LSB at the

analog input and adjust R1 until the ADC output code flickers

between 0111 1110 and 0111 1111.

Figure 24. ADC Error Adjust Circuit

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