AD7660AST datasheet(13/20 Pages) AD

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REV. 0

AD7660

–13–

When the source impedance of the driving circuit is low, the

AD7660 can be driven directly. Large source impedances will

significantly affect the ac performances, especially the total

harmonic distortion. The maximum source impedance depends

on the amount of total harmonic distortion (THD) that can be

tolerated. The THD degrades in function of the source imped-

ance and the maximum input frequency as shown in Figure 8.

INPUT FREQUENCY – kHz

–100

100

–95

–90

–85

–80

–75

–70

RS = 500

RS = 100

RS = 50

RS = 20

Figure 8. THD vs. Analog Input Frequency and

Input Resistance

Driver Amplifier Choice

Although the AD7660 is easy to drive, the driver amplifier needs

to meet at least the following requirements:

•

The driver amplifier and the AD7660 analog input circuit

have to be able together to settle for a full-scale step the

capacitor array at a 16-bit level (0.0015%). For instance,

operation at the maximum throughput of 100 kSPS requires

a minimum gain bandwidth product of 5 MHz.

•

The noise generated by the driver amplifier needs to be kept

as low as possible in order to preserve the SNR and transi-

tion noise performance of the AD7660. The noise coming

from the driver is filtered by the AD7660 analog input circuit

one-pole low-pass filter made by R1 and C2. For instance, a

driver with an equivalent input noise of 7 nV/

√Hz like the

AD8519 and configured as a buffer, thus with a noise gain of

+1, degrades the SNR by only 0.2 dB.

•

The driver needs to have a THD performance suitable to

that of the AD7660. TPC 8 gives the THD versus frequency

that the driver should preferably exceed.

The AD8519, OP162, or the OP184 meet these requirements

and are usually appropriate for almost all applications. As an

alternative, in very high-speed and noise-sensitive applications,

the AD829 with an external compensation capacitor of 82 pF

can be used. This capacitor should have good linearity as an

NPO ceramic or mica type. Moreover, the use of a noninverting

+1 gain arrangement is recommended and helps to obtain the

best signal-to-noise ratio.

Voltage Reference Input

The AD7660 uses an external 2.5 V voltage reference. The

voltage reference input REF of the AD7660 has a dynamic

input impedance. Therefore, it should be driven by a low

impedance 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 low ESR tanta-

lum capacitor and a 100 nF ceramic capacitor. Appropriate

value for the tantalum capacitor is 47

µF with the low-cost,

low-power ADR291 voltage reference or with the low-noise,

low-drift AD780 voltage reference. For applications using

multiple AD7660s, 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 which 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.

Power Supply

The AD7660 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 6. The AD7660 is inde-

pendent of power supply sequencing and thus free from supply

voltage induced latchup. Additionally, it is very insensitive to

power supply variations over a wide frequency range as shown in

Figure 9.

INPUT FREQUENCY – Hz

–80

10k

100k

–75

–70

–65

–60

–55

–50

Figure 9. PSRR vs. Frequency

POWER DISSIPATION VS. THROUGHPUT

The AD7660 automatically reduces its power consumption at

the end of each conversion phase. During the acquisition phase,

the operating currents are very low which allows a significant

power saving when the conversion rate is reduced as shown in

Figure 10. This feature makes the AD7660 ideal for very low-

power battery 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 rails (i.e.,

DVDD and DGND for all inputs except EXT/

INT, INVSYNC,

INVSCLK, RDC/SDIN, and OVDD or OGND for the last

four inputs.

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