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Preliminary Technical Data

AD7693

Rev. PrB | Page 15 of 24

If desired, smaller reference decoupling capacitor values down

to 2.2 μF can be used with a minimal impact on performance,

especially DNL.

SINGLE-TO-DIFFERENTIAL DRIVER

For applications using a single-ended analog signal, either

bipolar or unipolar, the ADA4941-1 single-ended-to-differential

driver allows for a differential input into the part. The

schematic is shown in

Regardless, there is no need for an additional lower value

ceramic decoupling capacitor (for example, 100 nF) between the

REF and GND pins.

Figure 29.

R1 and R2 set the attenuation ratio between the input range and

the ADC range (VREF). R1, R2, and CF will be chosen depending

on the desired input resistance, signal bandwidth, antialiasing

and noise contribution. For example, for the ±10 V range with a

4 kΩ impedance, R2 = 1 kΩ and R1 = 4 kΩ.

POWER SUPPLY

The AD7693 uses two power supply pins: a core supply, VDD,

and a digital input/output interface supply, VIO. VIO allows

direct interface with any logic between 1.8 V and VDD. To

reduce the supplies needed, the VIO and VDD pins can be tied

together. The AD7693 is independent of power supply sequencing

between VIO and VDD. Additionally, it is very insensitive to

power supply variations over a wide frequency range, as shown

R3 and R4 set the common mode on the IN− input, and R5 and

R6 set the common mode on the IN+ input of the ADC. The

common mode should be set close to VREF/2; however, if single

supply is desired, it can be set slightly above VREF/2 to provide

some headroom for the

Figure 30.

ADA4941-1

output stage. For example,

for the ±10 V range with a single supply, R3 = 8.45 kΩ, R4 =

11.8 kΩ, R5 = 10.5 kΩ, and R6 = 9.76 kΩ.

10000

FREQUENCY (kHz)

100

1000

VREF = VDD = 5V

AD7693

REF

GND

VDD

IN+

2.7nF

100nF

2.7nF

IN–

+5V REF

±10V, ±5V, ...

+5.2V

10µF

ADA4941

100nF

Figure 30. PSRR vs. Frequency

The AD7693 powers down automatically at the end of each

conversion phase and, therefore, the power scales linearly with

the sampling rate. This makes the part ideal for low sampling

rate (even a few hertz) and low battery-powered applications.

Figure 29. Single-Ended-to-Differential Driver Circuit

VOLTAGE REFERENCE INPUT

The AD7693 voltage reference input, REF, has a dynamic input

impedance and should therefore be driven by a low impedance

source with efficient decoupling between the REF and GND

pins, as explained in the

1000

0.1

0.001

SAMPLING RATE (SPS)

100

100k

10k

VDD = 5V

VIO

10000

100

0.01

Layout section.

When REF is driven by a very low impedance source, for

example, a reference buffer using the AD8031 or the AD8605, a

10 μF (X5R, 0805 size) ceramic chip capacitor is appropriate for

optimum performance.

If an unbuffered reference voltage is used, the decoupling value

depends on the reference used. For instance, a 22 μF (X5R,

1206 size) ceramic chip capacitor is appropriate for optimum

performance using low temperature drift ADR43x and ADR44x

references.

Figure 31. Operating Currents vs. Sample Rate

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