AD7664ASTZ datasheet(15/25 Pages) AD

15 / 25 page

REV.

–14–

AD7664

For instance, in a driver like the AD8021, with an equivalent

input noise of 2 nV/

√Hz and configured as a buffer, thus with a

noise gain of 1, the SNR degrades by 0.58 dB.

• The driver needs to have a THD performance suitable to that

of the AD7664. TPC 12 gives the THD versus frequency

that the driver should preferably exceed.

The AD8021 meets these requirements and is usually appropri-

ate 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 also 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.

Voltage Reference Input

The AD7664 uses an external 2.5 V voltage reference.

The voltage reference input REF of the AD7664 has a dynamic

input impedance; it should, therefore, be driven by a low imped-

ance source with an efficient decoupling between REF and

REFGND inputs. 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 REF and

REFGND inputs with minimum parasitic inductance. 47

µF is an

appropriate value for the tantalum capacitor when used with one

of the recommended reference voltages:

• The low noise, low temperature drift ADR421 and AD780

voltage references

• The low power ADR291 voltage reference

• The low cost AD1582 voltage reference

For applications using multiple AD7664s, it is more effective to

buffer the reference voltage with a low noise, very stable op amp

like the AD8031.

Care should also be taken with the reference temperature coeffi-

cient of the voltage reference that directly affects the full-scale

accuracy, if this parameter matters. For instance, a

±15 ppm/°C

tempco of the reference changes the full scale by

±1 LSB/°C.

VREF , as mentioned in the specification table, could be increased to

AVDD – 1.85 V. The benefit here is the increased SNR obtained

as a result of this increase. Since the input range is defined in

terms of VREF, this would essentially increase the range to make it

a 0 V to 3 V input range with an AVDD above 4.85 V. The

theoretical improvement as a result of this increase in reference is

1.58 dB (20 log [3/2.5]). Due to the theoretical quantization noise,

however, the observed improvement is approximately 1 dB. The

AD780 can be selected with a 3 V reference voltage.

Power Supply

The AD7664 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 5.25 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 5. The AD7664 is independent

of power supply sequencing 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 9.

POWER DISSIPATION VERSUS THROUGHPUT

Operating currents are very low during the acquisition phase, which

allows significant power savings when the conversion rate is

reduced, as shown in Figure 10. This power saving depends on the

mode used. In Impulse Mode, the AD7664 automatically reduces

its power consumption at the end of each conversion phase. This

feature makes the AD7664 ideal for very low power battery-

operated applications. It should be noted 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 supply rails (i.e., DVDD or

DGND for all inputs except EXT/

INT, INVSYNC, INVSCLK,

RDC/SDIN, and OVDD or OGND for these last four inputs).

–50

INPUT FREQUENCY – kHz

1000

–60

–70

–80

100

–55

–65

–75

Figure 9. PSRR vs. Frequency

100k

0.1

SAMPLING RATE – SPS

100k

100

10k

100

0.1

WARP/NORMAL

IMPULSE

Figure 10. Power Dissipation vs. Sample Rate

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