AD7294 datasheet(22/48 Pages) AD

22 / 48 page

Data Sheet

AD7294

Rev. I | Page 21 of 47

If the analog input signal to be sampled is bipolar, the internal

reference of the ADC can be used to externally bias up this

signal so that it is correctly formatted for the ADC. Figure 37

shows a typical connection diagram when operating the ADC

in single-ended mode.

VIN

+1.25V

–1.25V

REFOUT ADC

VIN0

AD7294

VIN3

+2.5V

0.47µF

1ADDITIONAL PINS OMITTED FOR CLARITY.

Figure 37. Single-Ended Mode Connection Diagram

Differential Mode

The AD7294 can have two differential analog input pairs.

Differential signals have some benefits over single-ended

signals, including noise immunity based on the common-

mode rejection of the device and improvements in distortion

performance. Figure 38 defines the fully differential analog

input of the AD7294.

VIN+

AD7294

VIN–

VREF p-p

COMMON-MODE

VOLTAGE

1ADDITIONAL PINS OMITTED FOR CLARITY.

Figure 38. Differential Input Definition

The amplitude of the differential signal is the difference

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

pair (VIN+ − VIN−). The resulting converted data is stored in twos

complement format in the result register. Simultaneously drive

VIN0 and VIN1 by two signals, each of amplitude VREF (or 2 ×

VREF, depending on the range chosen), that are 180° out of

phase. Assuming the 0 V to VREF range is selected, the amplitude

of the differential signal is, therefore, −VREF to +VREF peak-to-

peak (2 × VREF), regardless of the common mode (VCM).

The common mode is the average of the two signals

(VIN+ + VIN−)/2

The common mode is, therefore, the voltage on which the two

inputs are centered.

This results in the span of each input being VCM ± 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.

The common mode must be in this range to guarantee the

functionality of the AD7294.

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 output codes of −2048 to

+2047 in twos complement format.

If the 2 × VREF range is used, the input signal amplitude extends

from −2 ×VREF (VIN+ = 0 V, VIN− = VREF) to +2 × VREF (VIN− = 0 V,

VIN+ = VREF).

Driving Differential Inputs

The differential modes available on VIN0 to VIN3 in Table 13

require that VIN+ and VIN− be driven simultaneously with two

equal signals that are 180° out of phase. The common mode on

which the analog input is centered must be set up externally. The

common-mode range 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. Because not all applications have a signal preconditioned

for differential operation, there is often a need to perform a 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 AD7294. The circuit con-

figurations illustrated in Figure 39 show how a dual op amp can be

used to convert a single-ended bipolar signal into a differential

unipolar input signal.

The voltage applied to Point A sets up the common-mode voltage.

As shown in Figure 39, Point A connects to the reference, but any

value in the common-mode range can be the input at Point A 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 AD7294.

Care is required when choosing the op amp because the selection

depends on the required power supply and system performance

objectives. The driver circuits in Figure 39 are optimized for dc

coupling applications requiring best distortion performance.

The differential op amp driver circuit shown in Figure 39 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.

AD7294 Datasheet(PDF) 22 Page - Analog Devices

AD7294 Datasheet(HTML) 22 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Datasheet Download

Link URL