AD7934 datasheet(20/32 Pages) AD

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AD7933/AD7934

Rev. B | Page 20 of 32

VREF

p-p

VIN+

VIN–

VREF

p-p

*ADDITIONAL PINS OMITTED FOR CLARITY.

AD7933/

AD7934*

COMMON-MODE

VOLTAGE

Figure 24. Differential Input Definition

The amplitude of the differential signal is the difference

between the signals applied to the VIN+ and VIN− pins in each

differential pair (that is, VIN+ − VIN−). VIN+ and VIN− should be

simultaneously driven by two signals, each of amplitude

VREF (or 2 × VREF depending on the range chosen) that are

180° out of phase. The amplitude of the differential signal is,

therefore, −VREF to +VREF peak-to-peak (that is, 2 × VREF). This is

regardless of the common mode (CM). The common mode is

the average of the two signals (that is (VIN+ + VIN−)/2) and is,

therefore, the voltage on which the two inputs are centered.

This results in the span of each input being CM ± VREF/2. This

voltage has to be set up externally and its range varies with the

reference value, VREF. As the value of VREF increases, the

common-mode range decreases. When driving the inputs with

an amplifier, the actual common-mode range is determined by

the output voltage swing of the amplifier.

Figure 25 and Figure 26 show how the common-mode range

typically varies with VREF for a 5 V power supply using the 0 V

to VREF range or 2 × VREF range, respectively. The common

mode must be in this range to guarantee the functionality of the

AD7933/AD7934.

When a conversion takes place, the common mode is rejected,

resulting in a virtually noise-free signal of amplitude −VREF to

+VREF corresponding to the digital codes of 0 to 1024 for the

AD7933, and 0 to 4096 for the AD7934. If the 2 × VREF range is

used, the input signal amplitude extends from −2 VREF to

+2 VREF.

VREF (V)

3.5

3.0

2.0

1.5

2.5

1.0

0.5

00.5

1.5

1.0

2.0

2.5

3.0

TA = 25°C

Figure 25. Input Common-Mode Range vs. VREF

(0 V to VREF Range, VDD = 5 V)

VREF (V)

4.5

4.0

3.0

1.5

2.0

2.5

3.5

1.0

0.5

0.1

0.6

1.6

1.1

2.1

2.6

TA = 25°C

Figure 26. Input Common-Mode Range vs. VREF

(2 × VREF Range, VDD = 5 V)

Driving Differential Inputs

Differential operation requires that VIN+ and VIN− be simultane-

ously driven with two equal signals that are 180° out of phase.

The common mode must be set up externally and has a range

that is determined by VREF, the power supply, and the particular

amplifier used to drive the analog inputs. Differential modes of

operation with either an ac or dc input provide the best THD

performance over a wide frequency range. Since not all applica-

tions have a signal preconditioned for differential operation,

there is often a need to perform single-ended-to-differential

conversion.

Using an 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 AD7933/AD7934.

The circuit configurations shown in Figure 27 and Figure 28

show how a dual op amp converts a single-ended signal into a

differential signal for both 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, but any value in the common-mode range can be

input here to set up the common mode. The AD8022 is a

suitable dual op amp that can be used in this configuration to

provide differential drive to the AD7933/AD7934.

Take care when choosing the op amp; the selection depends on

the required power supply and system performance objectives.

The driver circuits in Figure 27 and Figure 28 are optimized for

dc coupling applications requiring best distortion performance.

The circuit configuration shown in Figure 27 is configured to

convert and level shift a single-ended, ground-referenced

(bipolar) signal to a differential signal centered at the VREF level

of the ADC.

The circuit in Figure 28 converts a unipolar, single-ended signal

into a differential signal.

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