AD7705 datasheet(23/44 Pages) AD

23 / 44 page

AD7705/AD7706

Rev. C | Page 23 of 44

On the AD7706, the voltages applied to the analog input

channels are referenced to the COMMON input. For example, if

AIN1(−) is 2.5 V and AD7705 is configured for unipolar

operation with a gain of 2 and a VREF of 2.5 V, the input voltage

range on the AIN1(+) input is 2.5 V to 3.75 V.

If AIN1(−) is 2.5 V and AD7705 is configured for bipolar mode

with a gain of 2 and a VREF of 2.5 V, the analog input range on

the AIN1(+) input is 1.25 V to 3.75 V (i.e., 2.5 V ± 1.25 V). If

AIN1(−) is at GND, the part cannot be configured for bipolar

ranges in excess of ±100 mV.

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 the input

signal conditioning, it simply changes the data output coding

and the points on the transfer function where calibrations occur.

REFERENCE INPUT

The AD7705/AD7706 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 parts are functional with VREF voltages

down to 1 V, but performance will be degraded because the output

noise, in terms of LSB size, is larger. REF IN(+) must 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 might result in gain errors on the part. In this case,

the sampling switch resistance is 5 kΩ typ, and the reference

capacitor, 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 gains of 16, 32, 64, and 128, it is 5.5 pF,

4.25 pF, 3.625 pF, and 3.3125 pF, respectively.

The output noise performance outlined in Table 5, Table 6,

Table 7, and Table 8 is for an analog input of 0 V, which

effectively removes the effect of noise on the reference. To

obtain the noise performance 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 degrades the performance of the

AD7705/AD7706. In applications 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 is removed because the application is

ratiometric.

Recommended reference voltage sources for the AD7705/

AD7706 with a VDD of 5 V include the AD780, REF43, and

REF192; the recommended reference sources for the AD7705/

AD7706 operated with a VDD of 3 V include the AD589 and

AD1580. It is generally recommended to decouple the output of

these references to reduce the noise level further.

DIGITAL FILTERING

The AD7705/AD7706 each contain an on-chip, low-pass digital

filter that processes the output of the Σ-Δ modulator. Therefore,

the parts not only provide the ADC function, but also provide a

level of filtering. There are a number of system differences when

the filtering function is provided in the digital domain, rather

than in the analog domain.

For example, because it occurs after the A/D conversion

process, digital filtering can remove noise injected during the

conversion process, whereas analog filtering cannot do this. In

addition, the digital filter can be made programmable far more

readily than the analog filter. Depending on the digital filter

design, this provides the user with the update rate.

On the other hand, analog filtering can remove noise

superimposed 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 have overrange

headroom built into the Σ-Δ modulator and digital filter that

allows overrange excursions of 5% above the analog input range.

If noise signals are larger than this, consider filtering the analog

input, or reducing the input channel voltage so that its full scale

is half that of the analog input channel full scale. This provides

an overrange capability greater than 100% at the expense of

reducing 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. However,

the input sampling on the part provides attenuation at multiples

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

bands occur around multiples of the sampling frequency, fS, as

defined in Table 23. Thus, the unattenuated bands occur at n × fS

(where n = 1, 2, 3 . . .). At these frequencies, there are frequency

bands ±f3 dB wide (f3 dB is the cutoff frequency of the digital filter)

at either side where noise passes unattenuated to the output.

AD7705 Datasheet(PDF) 23 Page - Analog Devices

AD7705 Datasheet(HTML) 23 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

Datasheet Download

Link URL