AD7731 Analog Devices, AD7731 Datasheet - Page 23

• Manufacturer Part Number: AD7731
• Description: Low Noise, High Throughput 24-Bit Sigma-Delta ADC
• Manufacturer: Analog Devices

Specifications of AD7731

• Resolution (bits): 24bit
• # Chan: 3
• Sample Rate: 5MSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Bip,Diff-Uni
• Ain Range: (2Vref/PGA Gain) p-p,(Vref/PGA Gain) p-p
• Adc Architecture: Sigma-Delta
• Pkg Type: DIP,SOIC,SOP

ANALOG INPUT
Analog Input Channels
The AD7731 has six analog input pins (labelled AIN1 to AIN6)
which can be configured as either three fully differential input
channels or five pseudo-differential input channels. Bits CH0,
CH1 and CH2 of the Mode Register configure the input chan-
nel arrangement and the channel selection is as outlined
previously in Table XIII. The input pairs (either differential
or pseudo-differential) provide programmable-gain, input chan-
nels 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 AIN3 and
AIN4 pins can also be reconfigured as two digital output port
bits, also controlled by the Mode Register.
A differential multiplexer switches one of the two input channels
to the on-chip buffer amplifier. 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 Register (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 25 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
on the common-mode range. Care must be taken in setting up
the common-mode voltage and input voltage range so that 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 AD7731 must be biased negative
with respect to system ground such that the analog input voltage
does not go within 1.2 V of AGND. Care should taken to en-
sure that the differential between either AV
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 applied to the input of
the on-chip programmable gain amplifier (PGA). The PGA can
handle seven different unipolar input ranges and seven 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 +20 mV, 0 mV to +40 mV,
REV. 0
REV. A
DD
– 0.95 V which also places restrictions
DD
or DV
DD
and this
–23–
0 mV to +80 mV, 0 mV to +160 mV, 0 mV to +320 mV, 0 mV
to +640 mV and 0 V to +1.28 V while the bipolar ranges are
the input to the on-chip PGA.
Bipolar/Unipolar Inputs
The analog inputs on the AD7731 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 respec-
tive AIN(–) input. For example, if AIN(–) is +2.5 V and the
AD7731 is configured for an analog input range of 0 mV to
+20 mV, the input voltage range on the AIN(+) input is +2.5 V
to +2.52 V. If AIN(–) is +2.5 V and the AD7731 is configured
for an analog input range of 1.28 V, the analog input range on
the AIN(+) input is +1.22 V to +3.78 V (i.e., 2.5 V
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 AD7731 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 mid-
scale voltage resulting in a code of 100...000 and a full-scale
input voltage resulting in a code of 111...111. When the AD7731 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.
Burnout Currents
The AD7731 contains two 100 nA constant current generators,
one source current from AV
AIN1(–) to AGND. The currents are switched to the selected
analog input pair. Both currents are either on or off depending
on the BO bit of the Mode Register. These currents can be used
in checking that a transducer is still operational before attempt-
ing to take measurements on that channel. If the currents are
turned on, allowed flow in the transducer, a measurement of the
input voltage on the analog input taken and the voltage mea-
sured is full scale then it indicates that the transducer has gone
open-circuit. If the voltage measured is 0 V, it indicates that the
transducer has gone open-circuit. For normal operation, these
burnout currents are turned off by writing a 0 to the BO bit.
The current sources work over the normal absolute input volt-
age range specifications.
20 mV, 40 mV, 80 mV, 160 mV, 320 mV, 640 mV,
1.28 V. These are the nominal ranges which should appear at
DD
to AIN(+) and one sink from
AD7731
1.28 V).