AD7723 datasheet(18/23 Pages) AD

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AD7723

–18–

REV. 0

Figures 32 and 33 show two such circuits for driving the AD7723.

Figure 32 is intended for use when the input signal is biased

about 2.5 V while Figure 33 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 33

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 AD8041/42 but has to be supplied from a +7.5 V/

–2.5 V supply. The AD8041/AD8042 will typically degrade

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

single +5 V supply as the AD7723.

AD8047

10k

220

220pF

VIN(+)

VIN(–)

REF1

REF2

AD7723

10nF

220nF

220

AIN =

BIASED

ABOUT 2.5V

RSOURCE

1 F

RIN

390

RFB

220

GAIN = 2

RFB/(RSOURCE + RIN)

Figure 32. Single-Ended to Differential Input Circuit for

Bipolar Mode Operation (Analog Input Biased About +2.5 V)

AD8047

RFB

220

220pF

VIN(+)

VIN(–)

REF1

REF2

AD7723

10nF

220nF

220

AIN =

BIASED

ABOUT

GROUND

1 F

RSOURCE

RIN

390

RBALANCE1

220

GAIN = 2

RFB/(RIN + RSOURCE)

RBALANCE1 = RBALANCE2

(RIN + RSOURCE)/(2

RFB)

RREF2 = RREF1

(RIN + RSOURCE)/RFB

RREF1

10k

RREF2

20k

RBALANCE2

Figure 33. 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 bandgap reference and a reference buffer circuit. The

block diagram of the reference circuit is shown in Figure 34.

The internal reference voltage is connected to REF1 through a

Ω resistor and is internally buffered to drive the analog

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

nal reference a 1

µF capacitor is required between REF1 and

AGND to decouple the bandgap noise. If the internal reference

is required to bias external circuits, use an external precision op

amp to buffer REF1.

COMPARATOR

REFERENCE

BUFFER

1 F

REF2

REF1

2.5V

REFERENCE

SWITCHED-CAP

DAC REFERENCED

AD7723

Figure 34. Reference Circuit Block Diagram

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

nal reference, the reference buffer in Figure 34 can be turned off

by grounding the REF1 pin and the external reference can be

applied directly to pin REF2. The AD7723 will accept an exter-

nal 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 refer-

ence. For example, a 1.25 V reference would make the bipolar

input span

±1 V, but would degrade SNR.

In all cases, since the REF2 voltage connects to the analog

modulator, a 220 nF and 10 nF capacitor must connect directly

from REF2 to AGND. The external capacitor provides the

charge required for the dynamic load presented at the REF2 pin

(See Figure 35).

10nF

220nF

4pF

REF2

SWITCHED-CAP

DAC REFERENCED

CLKIN

Figure 35. REF2 Equivalent Input Circuit

The AD780 is ideal to use as an external reference with the

AD7723. Figure 36 shows a suggested connection diagram.

Grounding Pin 8 on the AD780 selects the 3 V output mode.

AD780

AD7723

REF2

REF1

+5V

1 F

22nF

220nF

10nF

22 F

+VIN

TEMP

GND

O/P

SELECT

VOUT

TRIM

2.5V

NC = NO CONNECT

Figure 36. External Reference Circuit Connection

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