AD7705 datasheet(17/32 Pages) AD

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AD7705/AD7706

–17–

REV. A

Bipolar or unipolar options are chosen by programming the

B/U

bit of the Setup Register. This programs the channel for either

unipolar or bipolar operation. Programming the channel for

either unipolar or bipolar operation does not change any of the

input signal conditioning, it simply changes the data output

coding and the points on the transfer function where calibra-

tions occur.

REFERENCE INPUT

The AD7705/AD7706’s reference inputs, REF IN(+) and

REF IN(–), provide a differential reference input capability.

The common-mode range for these differential inputs is from

GND to VDD. The nominal reference voltage, VREF (REF IN(+) –

REF IN(–)), for specified operation, is +2.5 V for the AD7705/

AD7706 operated with a VDD of 5 V and +1.225 V for the

AD7705/AD7706 operated with a VDD of 3 V. The part is func-

tional with VREF voltages down to 1 V, but with degraded per-

formance as the output noise will, in terms of LSB size, be larger.

REF IN(+) must always be greater than REF IN(–) for correct

operation of the AD7705/AD7706.

Both reference inputs provide a high impedance, dynamic load

similar to the analog inputs in unbuffered mode. The maximum

dc input leakage current is

±1 nA over temperature, and source

resistance may result in gain errors on the part. In this case, the

sampling switch resistance is 5 k

Ω typ and the reference capaci-

tor (CREF) varies with gain. The sample rate on the reference

inputs is fCLKIN/64 and does not vary with gain. For gains of 1

and 2, CREF is 8 pF; for a gain of 16, it is 5.5 pF, for a gain of

32, it is 4.25 pF, for a gain of 64, it is 3.625 pF and for a gain of

128, it is 3.3125 pF.

The output noise performance outlined in Tables I through IV

is for an analog input of 0 V, which effectively removes the

effect of noise on the reference. To obtain the same noise per-

formance as shown in the noise tables over the full input range

requires a low noise reference source for the AD7705/AD7706.

If the reference noise in the bandwidth of interest is excessive, it

will degrade the performance of the AD7705/AD7706. In appli-

cations where the excitation voltage for the bridge transducer on

the analog input also derives the reference voltage for the part,

the effect of the noise in the excitation voltage will be removed

as the application is ratiometric. Recommended reference volt-

age sources for the AD7705 with a VDD of 5 V include the

AD780, REF43 and REF192, while the recommended reference

sources for the AD7705 operated with a VDD of 3 V include the

AD589 and AD1580. It is generally recommended to decouple

the output of these references in order to further reduce the

noise level.

DIGITAL FILTERING

The AD7705/AD7706 contains an on-chip low-pass digital filter

which processes the output of the part’s sigma-delta modulator.

Therefore, the part not only provides the analog-to-digital con-

version function but also provides a level of filtering. There are a

number of system differences when the filtering function is

provided in the digital domain rather than the analog domain

and the user should be aware of these.

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

process, it can remove noise injected during the conversion

process. Analog filtering cannot do this. Also, the digital filter

can be made programmable far more readily than an analog

filter. Depending on the digital filter design, this gives the user

the capability of programming cutoff frequency and output

update rate.

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 AD7705/AD7706

has overrange headroom built into the sigma-delta modulator

and digital filter, which allows overrange excursions of 5%

above the analog input range. If noise signals are larger than

this, consideration 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 reduc-

ing the dynamic range by 1 bit (50%).

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

sampling frequency fS (as defined in Table XV). 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 AD7705/AD7706’s digital filter is a low-pass filter with a

(sinx/x)

3 response (also called sinc3). The transfer function for

this filter is described in the z-domain by:

H(z)

− Z –N

− Z –1

and in the frequency domain by:

H( f )

SIN(N

×π × f /f

S )

SIN(

π× f /f

S )

where N is the ratio of the modulator rate to the output rate.

Phase Response:

∠H = –3 π (N –2)× f /f

S Rad

Figure 4 shows the filter frequency response for a cutoff fre-

quency of 15.72 Hz, which corresponds to a first filter notch

frequency of 60 Hz. The plot is shown from dc to 390 Hz. This

response is repeated at either side of the digital filter’s sample

frequency and at either side of multiples of the filter’s sample

frequency.

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 12 where the

output rate is 60 Hz, the first notch of the filter is at 60 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.

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