AD7951BSTZ Datasheet, PDF (20/32 Pages) Analog Devices – 14-Bit, 1 MSPS, Unipolar/Bipolar Programmable Input PulSAR ADC

AD7951
ANALOG INPUTS
Input Range Selection
In parallel mode and serial hardware mode, the input range is
selected by using the BIPOLAR (bipolar) and TEN (10 Volt
range) inputs. See Table 6 for pin details and the Hardware
Configuration section and Software Configuration section for
programming the mode selection with either pins or configuration
register. Note that when using the configuration register, the
BIPOLAR and TEN inputs are don’t cares and should be tied to
either high or low.
Input Structure
Figure 28 shows an equivalent circuit for the input structure of
the AD7951.
VCC
0V TO 5V
RANGE ONLY
AVDD
D1
IN+ OR IN–
C_PIN
D2
VEE
D3 _RIN
C_IN
D4
AGND
Figure 28. AD7951 Simplified Analog Input
The four diodes, D1 to D4, provide ESD protection for the analog
inputs, IN+ and IN−. Care must be taken to ensure that the analog
input signal never exceeds the supply rails by more than 0.3 V,
because this causes the diodes to become forward-biased and to
start conducting current. These diodes can handle a forward-
biased current of 120 mA maximum. For instance, these conditions
could eventually occur when the input buffer’s U1 supplies are
different from AVDD, VCC, and VEE. In such a case, an input
buffer with a short-circuit current limitation can be used to protect
the part although most op amps’ short circuit current is <100 mA.
Note that D3 and D4 are only used in the 0 V to 5 V range to
allow for additional protection in applications that are switching
from the higher voltage ranges.
This analog input structure allows the sampling of the differential
signal between IN+ and IN−. By using this differential input,
small signals common to both inputs are rejected as shown in
Figure 29, which represents the typical CMRR over frequency.
Data Sheet
For instance, by using IN− to sense a remote signal ground,
ground potential differences between the sensor and the local
ADC ground are eliminated.
100
90
80
70
60
50
40
30
20
10
0
1
10
100
1000
10000
FREQUENCY (kHz)
Figure 29. Analog Input CMRR vs. Frequency
During the acquisition phase for ac signals, the impedance of
the analog inputs, IN+ and IN−, can be modeled as a parallel
combination of Capacitor CPIN and the network formed by the
series connection of RIN and CIN. CPIN is primarily the pin
capacitance. RIN is typically 70 Ω and is a lumped component
comprised of serial resistors and the on resistance of the switches.
CIN is primarily the ADC sampling capacitor and depending on the
input range selected is typically 48 pF in the 0 V to 5 V range,
typically 24 pF in the 0 V to 10 V and ±5 V ranges and typically
12 pF in the ±10 V range. During the conversion phase, when the
switches are opened, the input impedance is limited to CPIN.
Since the input impedance of the AD7951 is very high, it can be
directly driven by a low impedance source without gain error.
To further improve the noise filtering achieved by the AD7951
analog input circuit, an external, one-pole RC filter between the
amplifier’s outputs and the ADC analog inputs can be used, as
shown in Figure 27. However, large source impedances signifi-
cantly affect the ac performance, especially total harmonic
distortion (THD). The maximum source impedance depends on
the amount of THD that can be tolerated. The THD degrades as
a function of the source impedance and the maximum input
frequency.
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