AD7938BCPZ datasheet(22/36 Pages) AD

22 / 36 page

AD7938/AD7939

Data Sheet

Rev. C | Page 22 of 36

220

Ω

10k

Ω

2 × VREF p-p

GND

440

Ω

220

Ω

220

Ω

20k

Ω

220

Ω

V+

V–

V+

V–

VIN+

VIN–

VREF

AD7938/

AD7939

0.47µF

3.75V

2.5V

1.25V

3.75V

2.5V

1.25V

Figure 28. Dual Op Amp Circuit to Convert a Single-Ended

Bipolar Signal into a Differential Unipolar Signal

10k

Ω

VREF p-p

VREF

GND

440

Ω

220

Ω

220

Ω

20k

Ω

220

Ω

V+

V–

V+

V–

VIN+

VIN–

VREF

AD7938/

AD7939

0.47µF

3.75V

2.5V

1.25V

3.75V

2.5V

1.25V

Figure 29. Dual Op Amp Circuit to Convert a Single-Ended Unipolar Signal

into a Differential Signal

Another method of driving the AD7938/AD7939 is to use the

AD8138 differential amplifier. The AD8138 can be used as a

single-ended-to-differential amplifier or as a differential-to-

differential amplifier. The device is as easy to use as an op amp

and greatly simplifies differential signal amplification and

driving.

Pseudo Differential Mode

The AD7938/AD7939 can have four pseudo differential pairs

(Pseudo Mode 1) or seven pseudo differential inputs (Pseudo

Mode 2) by setting the MODE0 and MODE1 bits in the control

pseudo differential pairs, VIN+ is connected to the signal source,

which must have an amplitude of VREF (or 2 × VREF depending

on the range chosen) to make use of the full dynamic range of

the part. A dc input is applied to the VIN− pin. The voltage

applied to this input provides an offset from ground or a pseudo

ground for the VIN+ input. In the case of the seven pseudo

differential inputs, the seven analog input signals inputs are

referred to a dc voltage applied to VIN7.

The benefit of pseudo differential inputs is that they separate

the analog input signal ground from the ADC ground, allowing

dc common-mode voltages to be cancelled. Typically, this range

can extend from −0.3 V to +0.7 V when VDD = 3 V or −0.3 V to

+1.8 V when VDD = 5 V. Figure 30 shows a connection diagram

for pseudo differential mode.

VIN+

VIN–

VREF

AD7938/

AD7939*

*ADDITIONAL PINS OMITTED FOR CLARITY.

VREF p-p

0.47µF

DC INPUT

VOLTAGE

Figure 30. Pseudo Differential Mode Connection Diagram

ANALOG INPUT SELECTION

As shown in Table 10, users can set up their analog input

configuration by setting the values in the MODE0 and MODE1

bits in the control register. Assuming the configuration has been

chosen, there are different ways of selecting the analog input to

be converted depending on the state of the SEQ and SHDW bits

in the control register.

Traditional Multichannel Operation (SEQ = SHDW = 0)

Any one of eight analog input channels or four pairs of channels

can be selected for conversion in any order by setting the SEQ

and SHDW bits in the control register to 0. The channel to be

converted is selected by writing to the address bits, ADD2 to

ADD0, in the control register to program the multiplexer prior

to the conversion. This mode of operation is that of a traditional

multichannel ADC where each data write selects the next

channel for conversion. Figure 31 shows a flowchart of this

mode of operation. The channel configurations are shown in

Table 10.

POWER ON

WRITE TO THE CONTROL REGISTER TO

SET UP OPERATING MODE, ANALOG INPUT

AND OUTPUT CONFIGURATION.

SET SEQ = SHDW = 0. SELECT THE DESIRED

CHANNEL TO CONVERT (ADD2 TO ADD0).

ISSUE CONVST PULSE TO INITIATE A CONVERSION

ON THE SELECTED CHANNEL.

INITIATE A READ CYCLE TO READ THE DATA

FROM THE SELECTED CHANNEL.

INITIATE A WRITE CYCLE TO SELECT THE NEXT

CHANNEL TO BE CONVERTED BY

CHANGING THE VALUES OF BITS ADD2 TO ADD0

IN THE CONTROL REGISTER. SEQ = SHDW = 0.

Figure 31. Traditional Multichannel Operation Flow Chart

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