AD7705/AD7706
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.
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.
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.
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.
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.
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.
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.
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 5ꢀ 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 100ꢀ at the expense of
reducing the dynamic range by 1 bit (50ꢀ).
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.
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.
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.
Rev. C | Page 23 of 44
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