AD7660 datasheet(13/21 Pages) AD

13 / 21 page

REV.

AD7660

–12–

Analog Input

Figure 6 shows an equivalent circuit of the input structure of the

AD7660.

OR INGND

AGND

AVDD

Figure 6. Equivalent Analog Input Circuit

The two diodes D1 and D2 provide ESD protection for the

analog inputs IN and INGND. Care must be taken to ensure that

the analog input signal never exceeds the supply rails by more than

0.3 V. This will cause these diodes to become forward-biased and

start conducting current. These diodes can handle a forward-

biased current of 100 mA maximum. For instance, these

conditions could eventually occur when the input buffer’s (U1)

supplies are different from AVDD. In such cases, an input buffer

with a short circuit current limitation can be used to protect the part.

This analog input structure allows the sampling of the differential

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

CMRR 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 is eliminated.

FREQUENCY – Hz

10k

10M

100k

Figure 7. Analog Input CMRR 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 3242

W 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. It has to be noted that the input

impedance of the AD7660, unlike other SAR ADCs, is not a pure

capacitance and thus, inherently reduces the kickback transient at

the beginning of the acquisition phase. The R1, C2 makes a one-

pole low-pass filter with a typical cutoff frequency of 820 kHz that

reduces undesirable aliasing effect and limits the noise.

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 (THD). The maximum source impedance

depends on the amount of THD that can be tolerated. The

THD degrades in function of the source impedance 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

Source 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

must be able, together, to settle for a full-scale step of

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

transition 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

4 nV/

÷Hz like the OP184 and configured as a buffer, thus

with a noise gain of +1, degrades the SNR by only 0.1 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 SNR degradation due to the amplifier is:

SNR

fN e

LOSS

ÁÁ

˜˜

784

log

()

–

where:

f–3 dB is the –3 dB input bandwidth in MHz of the AD7660

(0.82 MHz) or the cutoff frequency of the input filter if any

are used.

N is the noise factor of the amplifier (1 if in buffer configuration).

eN is the equivalent input noise voltage of the op amp in

nV/

÷Hz.

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