AD7938BCPZ datasheet(21/36 Pages) AD

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Data Sheet

AD7938/AD7939

Rev. C | Page 21 of 36

VREF

p-p

VIN+

VIN–

VREF

p-p

*ADDITIONAL PINS OMITTED FOR CLARITY.

AD7938/

AD7939*

COMMON-MODE

VOLTAGE

Figure 25. 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 amplifier’s output

voltage swing.

Figure 26 and Figure 27 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 AD7938/AD7939.

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 4096 for the

AD7938 and 0 to 1024 for the AD7939. If the 2 × VREF range is

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

after conversion.

VREF (V)

3.5

3.0

2.0

1.5

2.5

1.0

0.5

1.5

1.0

2.0

2.5

3.0

TA = 25°C

Figure 26. 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 27. Input Common-Mode Range vs. VREF (2 × VREF Range, VDD = 5 V)

Driving Differential Inputs

Differential operation requires that VIN+ and VIN− be

simultaneously 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 applications 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 AD7938/AD7939.

The circuit configurations shown in Figure 28 and Figure 29

show how a dual op amp can be used to convert 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. A suitable dual op amp

that can be used in this configuration to provide differential

drive to the AD7938/AD7939 is the AD8022.

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

the required power supply and system performance objectives.

The driver circuits in Figure 28 and Figure 29 are optimized for

dc coupling applications requiring best distortion performance.

The differential op amp driver circuit in Figure 28 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 configuration shown in Figure 29 converts a

unipolar, single-ended signal into a differential signal.

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