Datasheet

REV. G
–16–
AD7710
Bipolar/Unipolar Inputs
The two analog inputs on the AD7710 can accept either unipo-
lar or bipolar input voltage ranges. Bipolar or unipolar options
are chosen by programming the B/U bit of the control register.
This programs both channels for either type of operation.
Programming the part for either unipolar or bipolar operation
does not change any of the input signal conditioning; it sim-
ply changes the data output coding, using binary for unipolar
inputs and offset binary for bipolar inputs.
The input channels are differential and, as a result, the voltage
to which the unipolar and bipolar signals are referenced is the
voltage on the AIN(–) input. For example, if AIN(–) is 1.25 V
and the AD7710 is configured for unipolar operation with a
gain of 1 and a V
REF
of 2.5 V, the input voltage range on the
AIN(+) input is 1.25 V to 3.75 V. If AIN(–) is 1.25 V and
the AD7710 is configured for bipolar mode with a gain of 1
and a V
REF
of 2.5 V, the analog input range on the AIN(+)
input is –1.25 V to +3.75 V.
REFERENCE INPUT/OUTPUT
The AD7710 contains a temperature compensated 2.5 V refer-
ence which has an initial tolerance of ±1%. This reference volt-
age is provided at the REF OUT pin, and it can be used as the
reference voltage for the part by connecting the REF OUT pin
to the REF IN(+) pin. This REF OUT pin is a single-ended
output, referenced to AGND, which is capable of providing up
to 1 mA to an external load. In applications where REF OUT is
connected to REF IN(+), REF IN(–) should be tied to AGND
to provide the nominal 2.5 V reference for the AD7710.
The reference inputs of the AD7710, REF IN(+) and REF IN(–)
provide a differential reference input capability. The common-
mode range for these differential inputs is from V
SS
to AV
DD
.
The nominal differential voltage, V
REF
(REF IN(+) – REF IN(–)),
is 2.5 V for specified operation, but the reference voltage can go
to 5 V with no degradation in performance if the absolute value
of REF IN(+) and REF IN(–) does not exceed its AV
DD
and
V
SS
limits, and the V
BIAS
input voltage range limits are obeyed.
The part is also functional with V
REF
voltage down to 1 V but
with degraded performance because 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 AD7710.
Both reference inputs provide a high impedance, dynamic load
similar to the analog inputs. The maximum dc input leakage cur-
rent is 10 pA (±1 nA over temperature), and source resistance may
result in gain errors on the part. The reference inputs look like the
analog input (see Figure 7). In this case, R
INT
is 5 kΩ typ and C
INT
varies with gain. The input sample rate is f
CLK IN
/256 and does not
vary with gain. For gains of 1 to 8, C
INT
is 20 pF; for a gain of 16,
it is 10 pF; for a gain of 32, it is 5 pF; for a gain of 64, it is 2.5 pF;
and for a gain of 128, it is 1.25 pF.
The digital filter of the AD7710 removes noise from the reference
input just as it does with the analog input, and the same limita-
tions apply regarding lack of noise rejection at integer multiples
of the sampling frequency. The output noise performance
outlined in Tables I and II assumes a clean reference. If the
reference noise in the bandwidth of interest is excessive, it can
degrade the performance of the AD7710. Using the on-chip
reference as the reference source for the part (that is, connecting
REF OUT to REF IN) results in degraded output noise perfor-
mance from the AD7710 for portions of the noise table that are
dominated by the device noise. The on-chip reference noise
effect is eliminated in ratiometric applications where the refer-
ence is used to provide the excitation voltage for the analog
front end. The connection scheme, shown in Figure 8, is recom-
mended when using the on-chip reference. Recommended refer-
ence voltage sources for the AD7710 include the AD580 and
AD680 2.5 V references.
REF OUT REF IN (+)
AD7710
REF IN (–)
Figure 8. REF OUT/REF IN Connection
V
BIAS
Input
The V
BIAS
input determines at what voltage the internal analog
circuitry is biased. It essentially provides the return path for
analog currents flowing in the modulator and, as such, it should
be driven from a low impedance point to minimize errors.
For maximum internal headroom, the V
BIAS
voltage should be
set halfway between AV
DD
and V
SS
. The difference between
AV
DD
and (V
BIAS
+ 0.85 × V
REF
) determines the amount of
headroom the circuit has at the upper end, while the difference
between V
SS
and (V
BIAS
– 0.85 × V
REF
) determines the amount
of headroom the circuit has at the lower end. When choosing a
V
BIAS
voltage, ensure that it stays within prescribed limits. For
single 5 V operation, the selected V
BIAS
voltage must ensure that
V
BIAS
± 0.85 × V
REF
does not exceed AV
DD
or V
SS
or that the
V
BIAS
voltage itself is greater than V
SS
+ 2.1 V and less than
AV
DD
– 2.1 V. For single 10 V operation or dual ±5 V opera-
tion, the selected V
BIAS
voltage must ensure that V
BIAS
± 0.85 ×
V
REF
does not exceed AV
DD
or V
SS
, or that the V
BIAS
voltage
itself is greater than V
SS
+ 3 V or less than AV
DD
–3 V. For
example, with AV
DD
= 4.75 V, V
SS
= 0 V, and V
REF
= 2.5 V, the
allowable range for the V
BIAS
voltage is 2.125 V to 2.625 V.
With AV
DD
= 9.5 V, V
SS
= 0 V, and V
REF
= 5 V, the range for
V
BIAS
is 4.25 V to 5.25 V. With AV
DD
= +4.75 V, V
SS
= –4.75 V,
and V
REF
= +2.5 V, the V
BIAS
range is –2.625 V to +2.625 V.
The V
BIAS
voltage does have an effect on the AV
DD
power supply
rejection performance of the AD7710. If the V
BIAS
voltage tracks
the AV
DD
supply, it improves the power supply rejection from
the AV
DD
supply line from 80 dB to 95 dB. Using an external
Zener diode, connected between the AV
DD
line and V
BIAS
, as the
source for the V
BIAS
voltage gives the improvement in AV
DD
power supply rejection performance.