AD7714_15 Datasheet, PDF (21/40 Pages) Analog Devices – Signal Conditioning ADC

AD7714
Table XIV. Input Sampling Frequency vs. Gain
Gain
Input Sampling Freq (f_S)
for the AD7714-3. The part is functional with V_REF voltages
down to 1 V but with degraded performance as the output noise
will, in terms of LSB size, be larger. REF␣ IN(+) must always be
1
f_{CLK IN}/64 (38.4␣ kHz @ f_{CLK IN} = 2.4576␣ MHz)
greater than REF␣ IN(–) for correct operation of the AD7714.
2
2 × f_{CLK IN}/64 (76.8␣ kHz @ f_{CLK IN} = 2.4576␣ MHz)
Both reference inputs provide a high impedance, dynamic load
4
4 × f_{CLK IN}/64 (153.6␣ kHz @ f_{CLK IN} = 2.4576␣ MHz) _{similar to the analog inputs in unbuffered mode. The maxi-}
8
8 × f_{CLK IN}/64 (307.2␣ kHz @ f_{CLK IN} = 2.4576␣ MHz) _{mum dc input leakage current is ± 1 nA over temperature and}
16
8 × f_{CLK IN}/64 (307.2␣ kHz @ f_{CLK IN} = 2.4576␣ MHz) _{source resistance may result in gain errors on the part. In this}
2
32
8 × f_{CLK IN}/64 (307.2␣ kHz @ f_{CLK IN} = 2.4576␣ MHz) _{case, the sampling switch resistance is 5␣ kΩ typ and the refer-}
64
8 × f_{CLK IN}/64 (307.2␣ kHz @ f_{CLK IN} = 2.4576␣ MHz) _{ence capacitor (CREF) varies with gain. The sample rate on the}
128
8 × f_{CLK IN}/64 (307.2␣ kHz @ f_{CLK IN} = 2.4576␣ MHz) _{reference inputs is fCLK IN/64 and does not vary with gain. For}
Burnout Current
gains of 1 to 8, C_REF 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
The AD7714 contains two 1␣ µA currents, one source current
gain of 128, it is 3.3125 pF.
from AV_DD to AIN(+) and one sink from AIN(–) to AGND. The
currents are either both on or off depending on the BO bit of the
Mode Register. These currents can be used in checking that a
transducer has not burned out nor gone open-circuit before
attempting to take measurements on that channel. If the cur-
rents are turned on, allowed flow in the transducer, a measure-
ment of the input voltage on the analog input taken and the
voltage measured is full scale, it indicates that the transducer has
gone open-circuit; if the voltage measured is zero, it indicates
that the transducer has gone short-circuit. For normal opera-
tion, these burnout currents are turned off by writing a 0 to the
BO bit. For the source current to work correctly, the applied
voltage on AIN(+) should not go within 500␣ mV of AV_DD. For
the sink current to work correctly, the applied voltage on the
AIN(–) input should not go within 500␣ mV of AGND.
The output noise performance outlined in Tables I through IV
is for an analog input of 0 V and is unaffected by noise on the
reference. To obtain the same noise performance as shown in
the noise tables over the full input range requires a low noise
reference source for the AD7714. If the reference noise in the
bandwidth of interest is excessive, it will degrade the perfor-
mance of the AD7714. In applications where the excitation
voltage for the bridge transducer on the analog input also de-
rives 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 voltage sources for the
AD7714-5 and AD7714Y grade with AV_DD = 5 V include the
AD780, REF43 and REF192 while the recommended reference
sources for the AD7714-3 and AD7714Y with AV_DD = 3 V
include the AD589 and AD1580. It is generally recommended
Bipolar/Unipolar Inputs
to decouple the output of these references to further reduce the
The analog inputs on the AD7714 can accept either unipolar or _{noise level.}
bipolar input voltage ranges. Bipolar input ranges do not imply
that the part can handle negative voltages on its analog inputs,
DIGITAL FILTERING
since the analog input cannot go more negative than –30␣ mV to The AD7714 contains an on-chip low-pass digital filter which
ensure correct operation of the part. The input channels are
processes the output of the part’s sigma-delta modulator. There-
either fully differential or pseudo-differential (all other channels fore, the part not only provides the analog-to-digital conversion
referenced to AIN6). In either case, the input channels are
function but it also provides a level of filtering. There are a
arranged in pairs with an AIN(+) and AIN(–). As a result, the
number of system differences when the filtering function is
voltage to which the unipolar and bipolar signals on the AIN(+) provided in the digital domain rather than the analog domain
input are referenced is the voltage on the respective AIN(–)
and the user should be aware of these.
input. For example, if AIN(–) is +2.5␣ V and the AD7714 is
configured for unipolar operation with a gain of 2 and a V_REF of
+2.5␣ V, the input voltage range on the AIN(+) input is +2.5 V to
+3.75␣ V. If AIN(–) is +2.5␣ V and the AD7714 is configured for
bipolar mode with a gain of 2 and a V_REF of +2.5␣ V, the analog
input range on the AIN(+) input is +1.25␣ V to +3.75 V (i.e.,
2.5␣ V ± 1.25␣ V). If AIN(–) is at AGND, the part cannot be con-
figured for bipolar ranges in excess of ± 30␣ mV.
Bipolar or unipolar options are chosen by programming the B/U
bit of the Filter High Register. This programs the selected chan-
nel 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
calibrations occur.
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 AD7714 has over-
range headroom built into the sigma-delta modulator and digital
filter which allows overrange excursions of 5% above the analog
REFERENCE INPUT
The AD7714’s reference inputs, REF␣ IN(+) and REF␣ IN(–),
provide a differential reference input capability. The common-
mode range for these differential inputs is from AGND to AV_DD.
The nominal reference voltage, V_REF (REF␣ IN(+)␣ –REF␣ IN(–)),
for specified operation is +2.5␣ V for the AD7714-5 and +1.25␣ V
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 reducing the
dynamic range by 1 bit (50%).
REV. C
–21–