AD7723 datasheet(21/32 Pages) AD

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AD7723

Rev. C | Page 21 of 32

VIN(+)

VIN(–)

AD7723

Figure 37. Input RC Network

With the unipolar input mode selected, just one op amp is

required to buffer single-ended input signals. However, driving

the AD7723 with complementary signals and with the bipolar

input range selected has some distinct advantages: even-order

harmonics in both the drive circuits and the AD7723 front end

are attenuated and the peak-to-peak input signal range on both

inputs is halved. Halving the input signal range allows some op

amps to be powered from the same supplies as the AD7723.

Although a complementary driver requires the use of two op

amps per ADC, it may avoid the need to generate additional

supplies just for these op amps.

Figure 38 and Figure 39 show two such circuits for driving the

AD7723. Figure 38 is intended for use when the input signal is

biased about 2.5 V, while Figure 39 is used when the input signal

is biased about ground. While both circuits convert the input

signal into a complementary signal, the circuit in Figure 39 also

level shifts the signal so that both outputs are biased about 2.5 V.

Suitable op amps include the AD8047, AD8044, AD8041, and

its dual equivalent, the AD8042. The AD8047 has lower input

noise than the AD8041/AD8042 but has to be supplied from a

+7.5 V/−2.5 V supply. The AD8041/AD8042 typically degrades

SNR from 90 dB to 88 dB but can be powered from the same

single 5 V supply as the AD7723.

AD8047

RFB

220

Ω

220

Ω

220pF

VIN(+)

VIN(–)

REF1

REF2

AD7723

10nF

220nF

220

Ω

AIN =

±2V

BIASED

ABOUT 2.5V

µF

RSOURCE

Ω

RIN

390

Ω

GAIN = 2

× R

FB/(RSOURCE + RIN)

10k

Ω

Figure 38. Single-Ended-to-Differential Input Circuit for Bipolar Mode

Operation (Analog Input Biased About 2.5 V)

AD8047

RFB

220

Ω

220

Ω

220pF

VIN(+)

VIN(–)

REF1

REF2

AD7723

10nF

220nF

220

Ω

AIN =

±2V

BIASED

ABOUT

GROUND

µF

RSOURCE

Ω

RIN

390

Ω

RBALANCE1

220

Ω

GAIN = 2

× R

FB/(RIN + RSOURCE)

RBALANCE1 = RBALANCE2 × (RIN + RSOURCE)/(2 × RFB)

RREF2 = RREF1 × (RIN + RSOURCE)/RFB

RREF1

10k

Ω

RREF2

20k

Ω

RBALANCE2

Figure 39. Single-Ended-to-Differential Input Circuit for Bipolar Mode

Operation (Analog Input Biased About Ground)

APPLYING THE REFERENCE

The reference circuitry used in the AD7723 includes an on-chip

2.5 V band-gap reference and a reference buffer circuit. The

block diagram of the reference circuit is shown in Figure 40.

The internal reference voltage is connected to REF1 through a

3 kΩ resistor and is internally buffered to drive the analog

modulator’s switched cap DAC (REF2). When using the

internal reference, a 1 µF capacitor is required between REF1

and AGND to decouple the band-gap noise. If the internal

reference is required to bias external circuits, use an external

precision op amp to buffer REF1.

COMPARATOR

REFERENCE

BUFFER

µF

REF2

REF1

2.5V

REFERENCE

SWITCHED-CAP

DAC REFERENCED

AD7723

Ω

Figure 40. Reference Circuit Block Diagram

Where gain error or gain error drift requires the use of an

external reference, the reference buffer in Figure 40 can be

turned off by grounding the REF1 pin and the external

reference can be applied directly to REF2 pin. The AD7723

accepts an external reference voltage between 1.2 V to 3.15 V.

By applying a 3 V rather than a 2.5 V reference, SNR is typically

improved by about 1 dB. Where the output common-mode

range of the amplifier driving the inputs is restricted, the full-

scale input signal span can be reduced by applying a lower than

2.5 V reference. For example, a 1.25 V reference would make

the bipolar input span ±1 V but would degrade SNR.

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