AD7730_15 Datasheet, PDF (24/53 Pages) Analog Devices – Bridge Transducer ADC

AD7730/AD7730L
ANALOG INPUT
Analog Input Channels
The AD7730 contains two differential analog input channels, a
primary input channel, AIN1, and a secondary input channel,
AIN2. The input pairs provide programmable gain, differential
channels which can handle either unipolar or bipolar input
signals. It should be noted that the bipolar input signals are
referenced to the respective AIN(–) input of the input pair. The
secondary input channel can also be reconfigured as two digital
output port bits.
A two-channel differential multiplexer switches one of the two
input channels to the on-chip buffer amplifier. This multiplexer
is controlled by the CH0 and CH1 bits of the Mode Register.
When the analog input channel is switched, the RDY output
goes high and the settling time of the part must elapse before a
valid word from the new channel is available in the Data Regis-
ter (indicated by RDY going low).
Buffered Inputs
The output of the multiplexer feeds into a high impedance input
stage of the buffer amplifier. As a result, the analog inputs can
handle significant source impedances. This buffer amplifier has
an input bias current of 50 nA (CHP = 1) and 60 nA (CHP = 0).
This current flows in each leg of the analog input pair. The
offset current on the part is the difference between the input
bias on the legs of the input pair. This offset current is less than
10 nA (CHP = 1) and 30 nA (CHP = 0). Large source resis-
tances result in a dc offset voltage developed across the source
resistance on each leg, but matched impedances on the analog
input legs will reduce the offset voltage to that generated by the
input offset current.
Analog Input Ranges
The absolute input voltage range is restricted to between
AGND + 1.2 V to AV_DD – 0.95 V, which also places restrictions
on the common-mode range. Care must be taken in setting up
the common-mode voltage and input voltage range so these
limits are not exceeded, otherwise there will be a degradation in
linearity performance.
In some applications, the analog input range may be biased
either around system ground or slightly below system ground. In
such cases, the AGND of the AD7730 must be biased negative
with respect to system ground so the analog input voltage does
not go within 1.2 V of AGND. Care should taken to ensure that
the differential between either AV_DD or DV_DD and this biased
AGND does not exceed 5.5 V. This is discussed in more detail
in the Applications section.
Programmable Gain Amplifier
The output from the buffer amplifier is summed with the output
of the 6-bit Offset DAC before it is applied to the input of the
on-chip programmable gain amplifier (PGA). The PGA can
handle four different unipolar input ranges and four bipolar
ranges. With the HIREF bit of the Mode Register at 0 and a
+2.5 V reference (or the HIREF bit at 1 and a +5 V reference),
the unipolar ranges are 0 mV to +10 mV, 0 mV to +20 mV,
0 mV to +40 mV, and 0 mV to +80 mV, while the bipolar ranges
are ± 10 mV, ± 20 mV, ± 40 mV and ± 80 mV. These are the
nominal ranges that should appear at the input to the on-chip
PGA.
Offset DAC
The purpose of the Offset DAC is to either add or subtract an
offset so the input range at the input to the PGA is as close as
possible to the nominal. If the output of the 6-bit Offset DAC is
0 V, the differential voltage ranges that appear at the analog
input to the part will also appear at the input to the PGA. If,
however, the Offset DAC has an output voltage other than 0 V,
the input range to the analog inputs will differ from that applied
to the input of the PGA.
The Offset DAC has five magnitude bits and one sign bit. The
sign bit determines whether the value loaded to the five magni-
tude bits is added to or subtracted from the voltage at the ana-
log input pins. Control of the Offset DAC is via the DAC
Register which is discussed previously in the On-Chip Registers
section. With a 5 V reference applied between the REF IN pins,
the resolution of the Offset DAC is 2.5 mV with a range that
allows addition or subtraction of 77.5 mV. With a 2.5 V refer-
ence applied between the REF IN pins, the resolution of the
Offset DAC is 1.25 mV with a range that allows addition or
subtraction of 38.75 mV.
Following is an example of how the Offset DAC works. If the
differential input voltage range the user had at the analog input
pins was +20 mV to +30 mV, the Offset DAC should be pro-
grammed to subtract 20 mV of offset so the input range to the
PGA is 0 mV to +10 mV. If the differential input voltage range
the user had at the analog input pins was –60 mV to +20 mV,
the Offset DAC should be programmed to add 20 mV of offset so
the input range to the PGA is ± 40 mV.
Bipolar/Unipolar Inputs
The analog inputs on the AD7730 can accept either unipolar or
bipolar input voltage ranges. Bipolar input ranges do not imply
that the part can handle negative voltages with respect to system
ground on its analog inputs unless the AGND of the part is also
biased below system ground. Unipolar and bipolar signals on
the AIN(+) input are referenced to the voltage on the respective
AIN(–) input. For example, if AIN(–) is +2.5 V and the AD7730 is
configured for an analog input range of 0 to +10 mV with no
DAC offset correction, the input voltage range on the AIN(+)
input is +2.5 V to +2.51 V. Similarly, if AIN(–) is +2.5 V and the
AD7730 is configured for an analog input range of ±80 mV
with no DAC offset correction, the analog input range on the
AIN(+) input is +2.42 V to +2.58 V (i.e., 2.5 V ± 80 mV).
Bipolar or unipolar options are chosen by programming the B/U
bit of the Mode Register. This programs the selected channel
for either unipolar or bipolar operation. Programming the chan-
nel 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. When the AD7730 is configured for unipolar
operation, the output coding is natural (straight) binary with a
zero differential voltage resulting in a code of 000 . . . 000, a
midscale voltage resulting in a code of 100 . . . 000 and a full-
scale input voltage resulting in a code of 111 . . . 111. When the
AD7730 is configured for bipolar operation, the coding is offset
binary with a negative full scale voltage resulting in a code of
000 . . . 000, a zero differential voltage resulting in a code of
100 . . . 000 and a positive full scale voltage resulting in a code
of 111 . . . 111.
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REV. B