ADUC836 datasheet(27/80 Pages) AD

27 / 80 page

REV. A

ADuC834

–27–

In operation, the analog signal sample is fed to the difference

amplifier along with the output of the feedback DAC. The

difference between these two signals is integrated and fed to the

comparator. The output of the comparator provides the input to

the feedback DAC so the system functions as a negative feedback

loop that tries to minimize the difference signal. The digital

data that represents the analog input voltage is contained in the

duty cycle of the pulse train appearing at the output of the com-

parator. This duty cycle data can be recovered as a data-word

using a subsequent digital filter stage. The sampling frequency of

the modulator loop is many times higher than the bandwidth

of the input signal. The integrator in the modulator shapes the

quantization noise (which results from the analog-to-digital

conversion) so that the noise is pushed toward one-half of the

modulator frequency.

Digital Filter

The output of the

modulator feeds directly into the digital

filter. The digital filter then band-limits the response to a fre-

quency significantly lower than one-half of the modulator frequency.

In this manner, the 1-bit output of the comparator is translated

into a band-limited, low noise output from the ADuC834 ADCs.

The ADuC834 filter is a low-pass, Sinc

3 or (SIN x/x)3 filter

whose primary function is to remove the quantization noise

introduced at the modulator. The cutoff frequency and decimated

output data rate of the filter are programmable via the SF (Sinc

Filter) SFR as described in Table VIII.

Figure 11 shows the frequency response of the ADC channel at

the default SF word of 69 dec or 45H, yielding an overall output

update rate of just under 20 Hz.

It should be noted that this frequency response allows frequency

components higher than the ADC Nyquist frequency to pass

through the ADC, in some cases without significant attenuation.

These components may, therefore, be aliased and appear

in-band after the sampling process.

It should also be noted that rejection of mains-related frequency

components, i.e., 50 Hz and 60 Hz, is seen to be at a level of

>65 dB at 50 Hz and >100 dB at 60 Hz. This confirms the data

sheet specifications for 50 Hz/60 Hz Normal Mode Rejection

(NMR) at a 20 Hz update rate.

020

8090

100

110

FREQUENCY – Hz

–20

–40

–70

–80

–90

–100

–110

–120

–10

–30

–60

–50

Figure 11. Filter Response, SF = 69 dec

The response of the filter, however, will change with SF word as

can be seen in Figure 12, which shows >90 dB NMR at 50 Hz

and >70 dB NMR at 60 Hz when SF = 255 dec.

020

7080

110

FREQUENCY – Hz

–20

–40

–70

–80

–90

–100

–110

–120

–10

–30

–60

–50

Figure 12. Filter Response, SF = 255 dec

Figures 13 and 14 show the NMR for 50 Hz and 60 Hz across

the full range of SF word, i.e., SF = 13 dec to SF = 255 dec.

110

150 170 190 210

SF – Decimal

–20

–40

–70

–80

–90

–100

–110

–120

130

–10

–30

–60

–50

230 250

Figure 13. 50 Hz Normal Mode Rejection vs. SF

110

150 170 190 210

SF – Decimal

–20

–40

–70

–80

–90

–100

–110

–120

130

–10

–30

–60

–50

230 250

Figure 14. 60 Hz Normal Mode Rejection vs. SF

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