AD7712AR Analog Devices Inc, AD7712AR Datasheet - Page 16

• Manufacturer Part Number: AD7712AR
• Description: IC ADC SIGNAL COND LC2MOS 24SOIC
• Manufacturer: Analog Devices Inc

Specifications of AD7712AR

• Mounting Type: Surface Mount
• Rohs Status: RoHS non-compliant
• Number Of Bits: 24
• Sampling Rate (per Second): 1.03k
• Data Interface: Serial
• Number Of Converters: 1
• Power Dissipation (max): 45mW
• Voltage Supply Source: Analog and Digital, Dual ±
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 24-SOIC (0.300", 7.50mm Width)
• Peak Reflow Compatible (260 C): No
• No. Of Bits: 24 Bit
• Leaded Process Compatible: No
• Features: Signal Conditioning
• Compl. Anal. Front-end: No. Of Channels
• Interface Type: Serial
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

AD7712
ANALOG INPUT FUNCTIONS
Analog Input Ranges
The analog inputs on the AD7712 provide the user with consid-
erable flexibility in terms of analog input voltage ranges. One of
the inputs is a differential, programmable gain, input channel
that can handle either unipolar or bipolar input signals. The
common-mode range of this input is from V
that the absolute value of the analog input voltage lies between
V
single-ended, programmable gain, high level input that accepts
analog input ranges of 0 to +4
The dc input leakage current on the AIN1 input is 10 pA maxi-
mum at 25°C (± 1 nA over temperature). This results in a dc
offset voltage developed across the source impedance. However,
this dc offset effect can be compensated for by a combination of
the differential input capability of the part and its system cali-
bration mode. The dc input current on the AIN2 input depends
on the input voltage. For the nominal input voltage range of
± 10 V, the input current is ± 225 µA typ.
Burnout Current
The AIN1(+) input of the AD7712 contains a 4.5 µA current
source that can be turned on/off via the control register. This
current source can be used in checking that a transducer has not
burned out or gone open circuit before attempting to take mea-
surements on that channel. If the current is turned on and is
allowed to flow into the transducer and a measurement of the
input voltage on the AIN1 input is taken, it can indicate that the
transducer is not functioning correctly. For normal operation,
this burnout current is turned off by writing a 0 to the BO bit in
the control register.
Bipolar/Unipolar Inputs
The two analog inputs on the AD7712 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 unipolar or bipolar
operation. Programming the part for either unipolar or bipolar
operation does not change any of the input signal conditioning;
it simply changes the data output coding. The data coding is
binary for unipolar inputs and offset binary for bipolar inputs.
The AIN1 input channel is differential and, as a result, the
voltage to which the unipolar and bipolar signals are referenced
is the voltage on the AIN1(–) input. For example, if AIN1(–) is
1.25 V and the AD7712 is configured for unipolar operation
with a gain of 1 and a V
the AIN1(+) input is 1.25 V to 3.75 V. If AIN1(–) is 1.25 V and
the AD7712 is configured for bipolar mode with a gain of 1 and
a V
–1.25 V to +3.75 V. For the AIN2 input, the input signals are
referenced to AGND.
REFERENCE INPUT/OUTPUT
The AD7712 contains a temperature compensated 2.5 V refer-
ence, which has an initial tolerance of ± 1%. This reference
voltage is provided at the REF OUT, pin and 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
SS
REF
– 30 mV and AV
of 2.5 V, the analog input range on the AIN1(+) input is
DD
REF
+ 30 mV. The second analog input is a
of 2.5 V, the input voltage range on
V
REF
/GAIN or ±4
SS
to AV
V
REF
DD
/GAIN.
provided
–16–
to 1 mA to an external load. In applications where REF OUT
is connected directly to REF IN(+), REF IN(–) should be tied
to AGND to provide the nominal 2.5 V reference for the
AD7712.
The reference inputs of the AD7712, REF IN(+) and
REF IN(–), provide a differential reference input capability.
The common-mode range for these differential inputs is from
V
IN(+) – REF IN(–)), is 2.5 V for specified operation, but the
reference voltage can go to 5 V with no degradation in perfor-
mance provided that the absolute value of REF IN(+) and REF
IN(–) does not exceed its AV
input voltage range limits are obeyed. The part is also functional
with V
as the output noise will, in terms of LSB size, be larger. REF
IN(+) must always be greater than REF IN(–) for correct opera-
tion of the AD7712.
Both reference inputs provide a high impedance, dynamic load
similar to the AIN1 analog inputs. The maximum dc input
leakage current is 10 pA (± 1 nA over temperature), and source
resistance may result in gain errors on the part. The reference
inputs look like the AIN1 analog input (see Figure 7). In this
case, R
sample rate is f
of 1 to 8, C
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 AD7712 removes noise from the refer-
ence input just as it does with the analog input, and the same
limitations apply regarding lack of noise rejection at integer
multiples of the sampling frequency. The output noise perfor-
mance 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 AD7712. Using the on-chip
reference as the reference source for the part (i.e., connecting
REF OUT to REF IN) results in somewhat degraded output
noise performance from the AD7712 for portions of the noise
table that are dominated by the device noise. The on-chip refer-
ence noise effect is eliminated in ratiometric applications where
the reference is used to provide its excitation voltage for the analog
front end. The connection scheme shown in Figure 9 between
the REF OUT and REF IN pins of the AD7712 is recommended
when using the on-chip reference. Recommended reference
voltage sources for the AD7712 include the AD780 and AD680
2.5 V references.
SS
to AV
REF
INT
DD
voltages down to 1 V but with degraded performance
is 5 kΩ typ and C
Figure 9. REF OUT/REF IN Connection
INT
. The nominal differential voltage, V
CLK IN
is 20 pF; for a gain of 16, it is 10 pF; for a gain
REF OUT
AD7712
/256 and does not vary with gain. For gains
INT
DD
varies with gain. The input
and V
REF IN(+)
REF IN(–)
SS
limits and the V
REF
(REF
REV. F
BIAS