AD7714YN datasheet(22/40 Pages) AD

22 / 40 page

AD7714

REV. C

–22–

The cutoff frequency of the digital filter is determined by the

value loaded to bits FS0 to FS11 in the Filter High and Filter

Low Registers. Programming a different cutoff frequency via

FS0 – FS11 does not alter the profile of the filter response; it

changes the frequency of the notches as outlined in the Filter

Registers section. The output update and first notch correspond

and are determined by the relationship:

Output Rate = fCLK IN/(N.128)

where N is the decimal equivalent of the word loaded to the

FS0 to FS11 bits of the Filter Registers

while the –3 dB frequency is determined by the relationship:

–3 dB frequency = 0.262

× filter first notch frequency

The filter provides a linear phase response with a group delay

determined by:

Group Delay = –3

π.(N.f/f

MOD )

where N is the decimal equivalent of the word loaded to the

FS0 to FS11 bits of the Filter Registers and fMOD = fCLK IN/128.

Since the AD7714 contains this on-chip, low-pass filtering, a

settling time is associated with step function inputs and data on

the output will be invalid after a step change until the settling

time has elapsed. The settling time depends upon the output

rate chosen for the filter. The settling time of the filter to a full-

scale step input can be up to four times the output data period.

For a synchronized step input (using the

SYNC or FSYNC

functions) the settling time is three times the output data pe-

riod. When changing channels on the part, the change from one

channel to the other is synchronized so the output settling time

is also three times the output data period. Thus, in switching

between channels, the output data register is not updated until

the settling time of the filter has elapsed.

Post-Filtering

The on-chip modulator provides samples at a 19.2 kHz output

rate with fCLK IN at 2.4576 MHz. The on-chip digital filter

decimates these samples to provide data at an output rate that

corresponds to the programmed output rate of the filter. Since

the output data rate is higher than the Nyquist criterion, the

output rate for a given bandwidth will satisfy most application

requirements. However, there may be some applications that

require a higher data rate for a given bandwidth and noise per-

formance. Applications that need this higher data rate will

require some post-filtering following the part’s digital filter.

For example, if the required bandwidth is 7.86 Hz but the

required update rate is 100 Hz, the data can be taken from the

AD7714 at the 100 Hz rate giving a –3 dB bandwidth of

26.2 Hz. Post-filtering can be applied to this to reduce the

bandwidth and output noise, to the 7.86 Hz bandwidth level,

while maintaining an output rate of 100 Hz.

Post-filtering can also be used to reduce the output noise from

the device for bandwidths below 1.26 Hz. At a gain of 128 and

a bandwidth of 1.26 Hz, the output rms noise is 140 nV. This

is essentially device noise or white noise and since the input is

chopped, the noise has a primarily flat frequency response. By

reducing the bandwidth below 1.26 Hz, the noise in the result-

ant passband can be reduced. A reduction in bandwidth by a

factor of 2 results in a reduction of approximately 1.25 in the

output rms noise. This additional filtering will result in a

longer settling time.

In addition, the digital filter does not provide any rejection at

integer multiples of the digital filter’s sample frequency. How-

ever, the input sampling on the part provides attenuation at

multiples of the digital filter’s sampling frequency so that the

unattenuated bands actually occur around multiples of the input

sampling frequency fS (as defined in Table XIV). Thus, the

unattenuated bands occur at n

× f

S (where n = 1, 2, 3. . .). At

these frequencies, there are frequency bands,

±f

3 dB wide (f3 dB is

the cutoff frequency of the digital filter) at either side where

noise passes unattenuated to the output.

Filter Characteristics

The AD7714’s digital filter is a low-pass filter with a (sinx/x)

response (also called sinc

3). The transfer function for this filter

is described in the z-domain by:

H(z )

− Z −













and in the frequency domain by:

Sin N

f f

Sin

f f

()

(. .

)

=×

Figure 4 shows the filter frequency response for a cutoff

frequency of 2.62 Hz which corresponds to a first filter notch

frequency of 10 Hz. The plot is shown from dc to 65 Hz.

This response is repeated at either side of the input sampling

frequency and at either side of multiples of the input sampling

frequency.

FREQUENCY – Hz

–40

–60

–80

–100

–120

–140

–160

–180

–200

–220

–20

–240

Figure 4. Frequency Response of AD7714 Filter

The response of the filter is similar to that of an averaging filter

but with a sharper roll-off. The output rate for the digital filter

corresponds with the positioning of the first notch of the filter’s

frequency response. Thus, for the plot of Figure 4 where the

output rate is 10 Hz, the first notch of the filter is at 10 Hz. The

notches of this (sinx/x)

3 filter are repeated at multiples of the

first notch. The filter provides attenuation of better than 100 dB

at these notches. For the example given, if the first notch is at

10 Hz, there will be notches (and hence >100 dB rejection) at

both 50 Hz and 60 Hz.

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