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AD7575JPZ

LC2MOS 5 μs 8-Bit ADC with Track/Hold

Analog Devices 

AD7575

UNIPOLAR OPERATION

The basic operation for the AD7575 is in the unipolar single

supply mode. Figure 15 shows the circuit connections to achieve

this, while the nominal transfer characteristic for unipolar opera-

tion is given in Figure 16. Since the offset and full-scale errors

on the AD7575 are very small, in many cases it will not be nec-

essary to adjust out these errors. If calibration is required, the

procedure is as follows:

Offset Adjust

Offset error adjustment in single-supply systems is easily achiev-

able by means of the offset null facility of an op amp when used

as a voltage follower for the analog input signal, AIN. The op

amp chosen should be able to operate from a single supply and

allow a common-mode input voltage range that includes 0 V

(e.g., TLC271). To adjust for zero offset, the input signal

source is set to +4.8 mV (i.e., 1/2 LSB) while the op amp offset

is varied until the ADC output code flickers between 000 . . . 00

and 000 . . . 01.

Full-Scale Adjust

The full scale or gain adjustment is made by forcing the analog

input AIN to +2.445 V (i.e., Full-Scale Voltage –3/2 LSB). The

magnitude of the reference voltage is then adjusted until the

ADC output code flickers between 111 . . . 10 and 111. . . 11.

BIPOLAR OPERATION

The circuit of Figure 17 shows how the AD7575 can be config-

ured for bipolar operation. The output code provided by the

AD7575 is offset binary. The analog input voltage range is

± 5 V, although the voltage appearing at the AIN pin of the

AD7575 is in the range 0 V to +2.46 V. Figure 18 shows the

transfer function for bipolar operation. The LSB size is now

39.06 mV. Calibration of the bipolar operation is outlined be-

low. Once again, because the errors are small, it may not be

necessary to adjust them. To maintain specified performance

without the calibration, all resistors should be 0.1% tolerance

with R4 and R5 replaced by one 3.3 kΩ resistor and R2 and R3

replaced by one 2.5 kΩ resistor.

Offset Adjust

Offset error adjustment is achieved by applying an analog input

voltage of –4.9805 V (–FS +1/2 LSB). Resistor R3 is then

adjusted until the output code flickers between 000 . . . 00 and

000 . . . 01.

Full-Scale Adjust

Full-scale or gain adjustment is made by applying an analog

input voltage of +4.9414 V (+FS –3/2 LSB). Resistor R4 is then

adjusted until the output code flickers between 111 . . . 10 and

111. . . 11.

+5V

47␮F 0.1␮F

A

A

+5V

3.3k⍀

+2.46V

MAX

+1.23V

+

AD589

–

A

47␮F

A

0.1␮F

A

A

VDD

CLK

AIN

BUSY

AD7575 CS

VREF

RD

TP

AGND

DGND

+5V

RCLK

100k⍀, 1%

CCLK

100pF, 2%

D

CONTROL

INPUTS

+5V

DB7–DB0

DATA OUT

D

Figure 15. Unipolar Configuration

+5V

R8

3.3k⍀

+5V +

47␮F

A

+

AD589

–

A

INPUT

VOLTAGE R1

10k⍀

R2

2.2k⍀

R3

500⍀

47␮F 0.1␮F

A

A

+5V

TLC271

0.1␮F

A

VDD

VREF

CLK

BUSY

AD7575 CS

RD

AIN

TP

R4

500⍀

R5

3k⍀

A

A

R6

R7

2.5k⍀ 2.5k⍀

A

AGND DGND

A

D

Figure 17. Bipolar Configuration

+5V

RCLK

100k⍀, 1%

CCLK

100pF, 2%

D

+5V

DB7–DB0

DATA OUT

OUTPUT

CODE

11111111

11111110

11111101

FULL SCALE

TRANSITION

00000011

00000010

FS = 2VREF

1LSB

=

FS

256

00000001

00000000

1LSB 3LSBs

FS

2LSBs

FS –1LSB

AIN, INPUT VOLTAGE (IN TERMS OF LSBs)

Figure 16. Nominal Transfer Characteristic for

Unipolar Operation

REV. B

OUTPUT

CODE

111...111

111...110

100...010

100...001

–FS

100...000

011...111

011...110

000...001

000...000

–1/2LSB

+1/2LSB

AIN

+FS –1LSB

FS = 5V

1LSB

=

FS

256

Figure 18. Nominal Transfer Characteristic for

Bipolar Operation

–9–

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