AD7761 datasheet(48/76 Pages) AD

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Data Sheet

AD7761

Rev. A | Page 47 of 75

Modulator Chopping Frequency

Figure 67 plots two scenarios that relate to the chopping frequency

of the AD7761 modulator.

The AD7761 uses a chopping technique in the modulator similar

to that of a chopped amplifier to remove offset, offset drift, and

1/f noise. The AD7761 default chopping rate is fMOD/32. In pin

control mode, the chop frequency is hardwired to fMOD/32. In

SPI control mode, the user can select the chop frequency to be

either fMOD/32 or fMOD/8.

As shown in Figure 67, the stop band rejection of the digital

filter is reduced at frequencies that relate to even multiples of

the chopping frequency (fCHOP). All other out of band frequencies

(excluding those already discussed relating to the modulator

clock frequency, fMOD) are rejected by the stop band attenuation

of the digital filter. An out of band tone with a frequency in the

range of (2 × fCHOP ) ± f3dB, where f3dB is the filter bandwidth

employed, is attenuated to the envelope determined by the chop

frequency setting (see Figure 67), and aliased into the pass

band. Out of band tones near additional even multiples of fCHOP

(that is, N × fCHOP, where N is an even integer), are attenuated

and aliased in the same way.

Chopping at fMOD/32 offers the best performance for noise,

offset, and offset drift for the AD7761.

For ac performance it may be useful to select chopping at fMOD/8

as this moves the first chopping tone to a higher frequency.

However, chopping at fMOD/8 may lead to slightly degraded

noise (approximately 1 dB loss in dynamic range) and offset

performance compared to the default chop rate of fMOD/32.

Table 28 shows the aliasing achieved by different order anti-

aliasing filter options at the critical frequencies of fMOD/32 and

fMOD/8 for chop aliasing, fMOD/16 for modulator saturation, and

2 × fMOD for the first zone with 0 dB attenuation. It assumes the

corner frequency of the antialiasing filter is at fMOD/64, which is

just above the maximum input bandwidth that the AD7761

digital filter can pass when using a decimate by 32 filter setting.

Table 28. External Antialiasing Filter Attenuation

RC Filter

fMOD/32

(dB)

fMOD/16

(dB)

fMOD/8

(dB)

2 × fMOD

(dB)

First Order

−6

−12

−18

−42

Second Order

−12

−24

−36

−84

Third Order

−18

−36

−54

−126

Modulator Saturation Point

A Σ-Δ modulator can be considered a standard control loop,

employing negative feedback. The control loop works to ensure

that the average processed error signal is very small over time. It

uses an integrator to remember preceding errors and force the

mean error to be zero. As the input signal rate of change increases

with respect to the modulator clock, fMOD, a larger voltage feedback

error is processed. Above a certain frequency, the error begins

to saturate the modulator.

For the AD7761, the modulator may saturate for full-scale input

frequencies greater than fMOD/16 (as shown in Figure 68),

depending on the rate of change of input signal, input signal

amplitude, and reference input level. A half power input tone at

fMOD/8 can also cause the modulator to saturate. In applications

where there may be high amplitude and frequency out of band

tones, a first-order antialiasing filter is required with a −3 dB

corner frequency set at fMOD/16 to protect against modulator

saturation. For example, if operating the AD7761 at full speed

and using a decimation rate of ×32 to achieve an output data

rate of 256 kSPS, the modulator rate is equal to 8.192 MHz. In

this instance, to protect against saturation, set the antialiasing

filter −3 dB corner frequency to 512 kHz.

–3

–6

–9

–12

–15

–18

–21

–24

–27

0.0004

0.004

fIN/fMOD

0.04

0.4

MAX SIGNAL

FIRST ORDER (

fMOD/64)

Figure 68. Maximum Input Signal vs. Frequency

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