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AD7654

Rev. B | Page 17 of 28

The AD8021 meets these requirements and is usually appro-

priate for almost all applications. The AD8021 needs an

external compensation capacitor of 10 pF. This capacitor should

have good linearity as an NPO ceramic or mica type. The

AD8022 could be used where a dual version is needed and a

gain of +1 is used.

The AD829 is another alternative where high frequency

(above 100 kHz) performance is not required. In a gain of +1,

it requires an 82 pF compensation capacitor.

The AD8610 is another option where low bias current is needed

in low frequency applications.

Refer to Table 8 for some recommended op amps.

Table 8. Recommended Driver Amplifiers

Amplifier

Typical Application

ADA4841

Very low noise, low distortion, low power,

low frequency

AD829

Very low noise, low frequency

AD8021

Very low noise, high frequency

AD8022

Very low noise, high frequency, dual

AD8655/AD8656

Low noise, 5 V single supply, low power,

low frequency, single/dual

AD8610/AD8620

Low bias current, low frequency,

single/dual

VOLTAGE REFERENCE INPUT

The AD7654 requires an external 2.5 V reference. The reference

input should be applied to REF, REFA, and REFB. The voltage

reference input REF of the AD7654 has a dynamic input

impedance; it should therefore be driven by a low impedance

source with an efficient decoupling. This decoupling depends

on the choice of the voltage reference but usually consists of a

1 μF ceramic capacitor and a low ESR tantalum capacitor

connected to the REFA, REFB, and REFGND inputs with

minimum parasitic inductance. A value of 47 μF is an appro-

priate value for the tantalum capacitor when using one of the

recommended reference voltages:

• The low noise, low temperature drift AD780, AD361,

ADR421, and ADR431 voltage reference.

• The low cost AD1582 voltage reference.

For applications using multiple AD7654s with one voltage

reference source, it is recommended that the reference source

drives each ADC in a star configuration with individual

decoupling placed as close as possible to the REF/REFGND

inputs. Also, it is recommended that a buffer, such as the

AD8031/AD8032, be used in this configuration.

Care should be taken with the reference temperature coefficient

of the voltage reference, which directly affects the full-scale

accuracy if this parameter is applicable. For instance, a

15 ppm/°C tempco of the reference changes the full-scale

accuracy by 1 LSB/°C.

POWER SUPPLY

The AD7654 uses three sets of power supply pins: an analog

5 V supply AVDD, a digital 5 V core supply DVDD, and a

digital input/output interface supply OVDD. The OVDD

supply allows direct interface with any logic working between

2.7 V and DVDD + 0.3 V. To reduce the number of supplies

needed, the digital core (DVDD) can be supplied through a

simple RC filter from the analog supply, as shown in Figure 18.

The AD7654 is independent of power supply sequencing, once

OVDD does not exceed DVDD by more than 0.3 V, and thus

free from supply voltage induced latch-up. Additionally, it is

very insensitive to power supply variations over a wide

frequency range, as shown in Figure 20.

FREQUENCY (kHz)

100

1000

10000

Figure 20. PSRR vs. Frequency

POWER DISSIPATION

In impulse mode, the AD7654 automatically reduces its power

consumption at the end of each conversion phase. During the

acquisition phase, the operating currents are very low, which

allows significant power savings when the conversion rate is

reduced, as shown in Figure 21. This feature makes the AD7654

ideal for very low power battery applications.

Note that the digital interface remains active even during the

acquisition phase. To reduce the operating digital supply

currents even further, the digital inputs need to be driven close

to the power rails (that is, DVDD and DGND), and OVDD

should not exceed DVDD by more than 0.3 V.

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