AD7352

Rev. 0 | Page 14 of 20

DRIVING DIFFERENTIAL INPUTS

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

AD7352.

simultaneously with two equal signals that are 180° out of

phase. Because not all applications have a signal preconditioned

for differential operation, there is often a need to perform a

single-ended-to-differential conversion.

GND

27Ω

27Ω

V+

V–

V+

V–

1.024V

2.048V

0V

1.024V

2.048V

0V

AD7352*

220Ω

10µF

*ADDITIONAL PINS OMITTED FOR CLARITY.

220Ω

220Ω

10kΩ

A

440Ω

Differential Amplifier

An ideal method of applying differential drive to the AD7352

is to use a differential amplifier such as the AD8138. This part

can be used as a single-ended-to-differential amplifier or as a

differential-to-differential amplifier. The AD8138 also provides

common-mode level shifting. Figure 20 shows how the AD8138

can be used as a single-ended-to-differential amplifier. The

positive and negative outputs of the AD8138 are connected to

the respective inputs on the ADC via a pair of series resistors

to minimize the effects of switched capacitance on the front end

of the ADC. The architecture of the AD8138 results in outputs

that are very highly balanced over a wide frequency range

without requiring tightly matched external components.

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

into a Differential Signal

GND

27Ω

27Ω

V+

V–

V+

V–

1.024V

2.048V

0V

1.024V

2.048V

0V

AD7352*

220Ω

10µF

*ADDITIONAL PINS OMITTED FOR CLARITY.

220Ω

220Ω

10kΩ

A

440Ω

20kΩ

220Ω

+2.048V

GND

–2.048V

AD8138

2.048V

1.024V

0V

2.048V

1.024V

0V

AD7352

10kΩ

10kΩ

10µF

*MOUNT AS CLOSE TO THE AD7352 AS POSSIBLE

51Ω

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

a Differential Unipolar Signal

ADC TRANSFER FUNCTION

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

The output coding for the AD7352 is straight binary. The designed

code transitions occur at successive LSB values (1 LSB, 2 LSBs,

characteristic is shown in Figure 23.

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

as each other. If the source has a 50 Ω impedance and a 50 Ω

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.

000 ... 000

000 ... 001

000 ... 010

111 ... 101

111 ... 110

111 ... 111

ANALOG INPUT

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 AD7352. The

circuit configurations 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 both a bipolar and unipolar input signal,

respectively.

Figure 23. AD7352 Ideal Transfer Characteristic