AD7713 datasheet(14/28 Pages) AD

14 / 28 page

REV. C

–14–

AD7713

DIGITAL FILTERING

The AD7713’s digital filter behaves like a similar analog filter,

with a few minor differences.

First, since digital filtering occurs after the A-to-D conversion

process, it can remove noise injected during the conversion pro-

cess. Analog filtering cannot do this.

On the other hand, analog filtering can remove noise superim-

posed on the analog signal before it reaches the ADC. Digital

filtering cannot do this, and noise peaks riding on signals near

full scale have the potential to saturate the analog modulator

and digital filter, even though the average value of the signal is

within limits. To alleviate this problem, the AD7713 has over-

range headroom built into the sigma-delta modulator and digi-

tal filter which allows overrange excursions of 5% above the

analog input range. If noise signals are larger than this, consid-

eration should be given to analog input filtering, or to reducing

the input channel voltage so that its full scale is half that of the

analog input channel full scale. This will provide an overrange

capability greater than 100% at the expense of reducing the dy-

namic range by 1 bit (50%).

Filter Characteristics

The cutoff frequency of the digital filter is determined by the

value loaded to bits FS0 to FS11 in the control register. At the

maximum clock frequency of 2 MHz, the minimum cutoff fre-

quency of the filter is 0.52 Hz while the maximum program-

mable cutoff frequency is 53.9 Hz.

Figure 6 shows the filter frequency response for a cutoff fre-

quency of 0.52 Hz which corresponds to a first filter notch fre-

quency of 2 Hz. This is a (sinx/x)

3 response (also called sinc3)

FREQUENCY – Hz

–20

–240

012

–160

–180

–200

–220

–80

–140

–40

–60

–120

–100

Figure 6. Frequency Response of AD7713 Filter

that provides >100 dB of 50 Hz and 60 Hz rejection. Program-

ming 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 Control Register section.

Since the AD7713 contains this on-chip, low-pass filtering,

there is a settling time 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

notch frequency chosen for the filter. The output data rate

equates to this filter notch frequency, and the settling time of

the filter to a full-scale step input is four times the output data

period. In applications using both input channels, the settling

time of the filter must be allowed to elapse before data from the

second channel is accessed.

Post Filtering

The on-chip modulator provides samples at a 3.9 kHz output

rate. The on-chip digital filter decimates these samples to pro-

vide data at an output rate which corresponds to the pro-

grammed first notch frequency of the filter. Since the output

data rate exceeds the Nyquist criterion, the output rate for a

given bandwidth will satisfy most application requirements.

However, there may be some applications which require a

higher data rate for a given bandwidth and noise performance.

Applications that need this higher data rate will require some

post filtering following the digital filter of the AD7713.

For example, if the required bandwidth is 1.57 Hz but the re-

quired update rate is 20 Hz, the data can be taken from the

AD7713 at the 20 Hz rate giving a –3 dB bandwidth of

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

width and output noise, to the 1.57 Hz bandwidth level, while

maintaining an output rate of 20 Hz.

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

the device for bandwidths below 0.52 Hz. At a gain of 128, the

output rms noise is 250 nV. This is essentially device noise or

white noise, and since the input is chopped, the noise has a flat

frequency response. By reducing the bandwidth below 0.52 Hz,

the noise in the resultant passband can be reduced. A reduction

in bandwidth by a factor of two results in a

√2 reduction in the

output rms noise. This additional filtering will result in a longer

settling time.

Antialias Considerations

The digital filter does not provide any rejection at integer mul-

tiples of the modulator sample frequency (n

× 3.9 kHz, where

n = 1, 2, 3 . . . ). This means that there are frequency bands,

±f

3 dB wide (f3 dB is cutoff frequency selected by FS0 to FS11)

where noise passes unattenuated to the output. However, due to

the AD7713’s high oversampling ratio, these bands occupy only

a small fraction of the spectrum and most broadband noise is

filtered. In any case, because of the high oversampling ratio a

simple, RC, single pole filter is generally sufficient to attenuate

the signals in these bands on the analog input and thus provide

adequate antialiasing filtering.

If passive components are placed in front of the AIN1 and AIN2

inputs of the AD7713, care must be taken to ensure that the

source impedance is low enough so as not to introduce gain er-

rors in the system. The dc input impedance for the AIN1 and

AIN2 inputs is over 1 G

Ω. The input appears as a dynamic load

that varies with the clock frequency and with the selected gain

(see Figure 7). The input sample rate, as shown in Table III,

determines the time allowed for the analog input capacitor, CIN,

to be charged. External impedances result in a longer charge

time for this capacitor, and this may result in gain errors being

AIN

HIGH

IMPEDANCE

> 1G

Ω

RINT (7kΩ typ)

CINT

(11.5pF typ)

SWITCHING FREQ DEPENDS ON

fCLK IN AND SELECTED GAIN

VBIAS

Figure 7. AIN1, AIN2 Input Impedance

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