REV. 0

AD7664

–12–

This analog input structure allows the sampling of the differen-

tial signal between IN and INGND. Unlike other converters,

the INGND input is sampled at the same time as the IN input.

By using this differential input, small signals common to both

inputs are rejected, as shown in Figure 7, which represents the

typical CMR over frequency. For instance, by using INGND to

sense a remote signal ground, difference of ground potentials

between the sensor and the local ADC ground are eliminated.

70

1

FREQUENCY – kHz

1k

50

30

0

100

60

40

20

10

10

Figure 7. Analog Input CMR vs. Frequency

During the acquisition phase, the impedance of the analog input

IN can be modeled as a parallel combination of capacitor C1

and the network formed by the series connection of R1 and C2.

Capacitor C1 is primarily the pin capacitance. The resistor R1 is

typically 140

Ω and is a lumped component made up of some

serial resistors and the on resistance of the switches. The capacitor

C2 is typically 60 pF and is mainly the ADC sampling capacitor.

During the conversion phase, where the switches are opened, the

input impedance is limited to C1. The R1, C2 makes a one-pole

low-pass filter that reduces undesirable aliasing effect and limits

the noise.

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

AD7664 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.

–70

10

FREQUENCY – kHz

1k

–80

–90

–100

100

–75

–85

–95

Figure 8. THD vs. Analog Input Frequency and

Source Resistance

Driver Amplifier Choice

Although the AD7664 is easy to drive, the driver amplifier needs

to meet at least the following requirements:

•

The driver amplifier and the AD7664 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 570 kSPS requires

a minimum gain bandwidth product of 39 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 AD7664. The noise coming

from the driver is filtered by the AD7664 analog input circuit

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

driver such as the AD829, with an equivalent input noise of

2nV/

√Hz and configured as a buffer, thus, with a noise gain

of 1, degrades the SNR by only 0.45 dB. A driver amplifier

with an equivalent input noise of 5 nV/

√Hz in the same con-

figuration will add 1.9 dB degradation.

•

To even further reduce the noise filtering done by the AD7664

analog input circuit, an external simple one-pole RC filter

between the amplifier output and the ADC analog input will

slightly improve the ac performances, specially, the SNR and

the transition noise. For example, as shown in Figure 5, a 15

Ω

source resistor with a 2.7 nF good linearity capacitor (NPO or

mica type) limit the bandwidth to 4 MHz.

•

The driver needs to have a THD performance suitable to that

of the AD7664. TPC 10 gives the THD versus frequency

that the driver should preferably exceed. The AD829 meets

these requirements. The AD829 requires an external compensa-

tion capacitor of 82 pF. This capacitor should have good

linearity as an NPO ceramic or mica or prolypropylene 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 AD7664 uses an external 2.5 V voltage reference. The voltage

reference input REF of the AD7664 has a dynamic input imped-

ance. 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 tantalum 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 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 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 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