AD7357 datasheet(14/20 Pages) AD

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AD7357

Preliminary Technical Data

Rev. PrF | Page 14 of 20

If the analog inputs source being used has zero impedance, all

four resistors (RG1, RG2, RF1, and RF2) should be the same. If the

source has a 50 Ω impedance and a 50 Ω termination, for

example, the value of RG2 should be increased by 25 Ω to

balance this parallel impedance on the input and thus ensure

that both the positive and negative analog inputs have the same

gain. The outputs of the amplifier are perfectly matched

balanced differential outputs of identical amplitude and are

exactly 180° out of phase.

+2.048V

GND

–2.048V

AD8138

RG1

RS*

RG2

RF2

VOCM

RF1

2.048V

VIN+

VIN–

1.024V

2.048V

1.024V

AD7357

CF2

CF1

10kΩ

10µF

REFA/REFB

*MOUNT AS CLOSE TO THE AD7357 AS POSSIBLE

AND ENSURE HIGH PRECISION RS RESISTORS ARE USED.

RS – 33Ω; RG1 = RF1 = RF2 = 499Ω; CF1 = CF2 = 39pF;

RG2 = 523Ω.

51Ω

Figure 20. Using the AD8138 as a Single-Ended-to-Differential Amplifier

Op Amp Pair

An op amp pair can be used to directly couple a differential

signal to one of the analog input pairs of the AD7357. The

circuit configurations shown in Figure 21 and Figure 22 show

how an op amp pair can be used to convert a single-ended

signal into a differential signal for a bipolar and unipolar input

signal, respectively. The voltage applied to Point A sets up the

common-mode voltage. In both diagrams, it is connected in

some way to the reference. The AD8022 is a suitable dual op

amp that could be used in this configuration to provide

differential drive to the AD7357.

GND

2 × VREF p-p

27Ω

V+

V–

V+

V–

VREF

1.024V

2.048V

1.024V

2.048V

REFA/REFB

VIN+

AD7357*

VIN–

220Ω

10µF

*ADDITIONAL PINS OMITTED FOR CLARITY.

220Ω

10kΩ

440Ω

Figure 21. Dual Op Amp Circuit to Convert a Single-Ended Unipolar Signal

into a Differential Signal

GND

2 × VREF p-p

27Ω

V+

V–

V+

V–

VREF

1.024V

2.048V

1.024V

2.048V

REFA/REFB

VIN+

AD7357*

VIN–

220Ω

10µF

*ADDITIONAL PINS OMITTED FOR CLARITY.

220Ω

10kΩ

440Ω

20kΩ

Figure 22. Dual Op Amp Circuit to Convert a Single-Ended Bipolar Signal into

a Differential Unipolar Signal

ADC TRANSFER FUNCTION

The output coding for the AD7357 is straight binary. The

designed code transitions occur at successive LSB values (such

as, 1 LSB, 2 LSBs). The LSB size is (2 × VREF)/16384. The ideal

transfer characteristic of the AD7357 is shown in Figure 23.

000 ... 000

000 ... 001

000 ... 010

111 ... 101

111 ... 110

111 ... 111

ANALOG INPUT

–VREF + 0.5 LSB

–VREF + 1 LSB

+VREF – 1.5 LSB

+VREF – 1 LSB

Figure 23. AD7357 Deal Transfer Characteristic

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