AD7678ASTZ datasheet(19/29 Pages) AD

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AD7678

Rev. A | Page 18 of 28

The AD8021 meets these requirements and is usually appropri-

ate for almost all applications. The AD8021 needs a 10 pF

external compensation capacitor, which should have good

linearity as an NPO ceramic or mica type.

The AD8022 could be used if a dual version is needed and gain

of 1 is present. The AD829 is an alternative in applications

where high frequency (above 100 kHz) performance is not

required. In gain of 1 applications, it requires an 82 pF

compensation capacitor. The AD8610 is another option when

low bias current is needed in low frequency applications.

Single-to-Differential Driver

For applications using unipolar analog signals, a single-ended-

to-differential driver will allow for a differential input into the

part. The schematic is shown in Figure 24. When provided an

input signal of 0 to VREF, this configuration will produce a

differential ±VREF with midscale at VREF/2.

If the application can tolerate more noise, the AD8138

differential driver can be used.

8.25k



2.5V

AD8021

590



AD7678

IN+

IN– REF

ANALOG INPUT

(UNIPOLAR

0V TO 4.096V)

10pF

AD8021

590



10pF

F

100nF

1.82k



REFBUFIN

03084-0-030

Figure 24. Single-Ended-to-Differential Driver Circuit

(Internal Reference Buffer Used)

Voltage Reference

The AD7678 allows the use of an external voltage reference with

or without the internal reference buffer.

Using the internal reference buffer is recommended when

sharing a common reference voltage between multiple ADCs is

desired.

However, the advantages of using the external reference voltage

directly are



The SNR and dynamic range improvement (about 1.7 dB)

resulting from the use of a reference voltage very close to

the supply (5 V) instead of a typical 4.096 V reference

when the internal buffer is used.



The power saving when the internal reference buffer is

powered down (PDBUF HIGH).

To use the internal reference buffer, PDBUF should be LOW. A

2.5 V reference voltage applied on the REFBUFIN input will

result in a 4.096 V reference on the REF pin.

In both cases, the voltage reference input REF has a dynamic

input impedance and therefore requires an efficient decoupling

between REF and REFGND inputs. The decoupling consists of

a low ESR 47 μF tantalum capacitor connected to the REF and

REFGND inputs with minimum parasitic inductance.

Care should also be taken with the reference temperature

coefficient of the voltage reference, which directly affects the

full-scale accuracy if this parameter matters. For instance, a

±4 ppm/°C temperature coefficient of the reference changes the

full scale by ±1 LSB/°C.

Power Supply

The AD7678 uses three sets of power supply pins: an analog 5 V

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

output interface supply (OVDD). The OVDD supply defines

the output logic level and 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 21. The AD7678 is independent of power

supply sequencing once OVDD does not exceed DVDD by

more than 0.3 V, and is therefore 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 25.

FREQUECY (kHz)

100

1000

10000

110

03084-0-031

Figure 25. PSRR vs. Frequency

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