EVAL-AD7612CB Analog Devices Inc, EVAL-AD7612CB Datasheet - Page 20

• Manufacturer Part Number: EVAL-AD7612CB
• Description: BOARD EVALUATION AD7612
• Manufacturer: Analog Devices Inc
• Series: PulSAR®r

Specifications of EVAL-AD7612CB

• Number Of Adc's: 1
• Number Of Bits: 16
• Sampling Rate (per Second): 750k
• Data Interface: Serial, Parallel
• Inputs Per Adc: 1 Single Ended
• Input Range: ±5 V, ±10 V
• Power (typ) @ Conditions: 205mW @ 750kSPS
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD7612
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD7612
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 AD7612.
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.
IN+ OR IN–
VEE
Figure 28. AD7612 Simplified Analog Input
C
PIN
VCC
D1
D2
RANGE ONLY
0 TO 5V
AVDD
D3
D4
R
AGND
IN
C
IN
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For instance, by using IN− to sense a remote signal ground,
ground potential differences between the sensor and the local
ADC ground are eliminated.
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 C
series connection of R
itance. R
of serial resistors and the on resistance of the switches. C
marily 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 C
Since the input impedance of the AD7612 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 AD7612 analog
input circuit, an external, one-pole RC filter between the ampli-
fier’s outputs and the ADC analog inputs can be used, as shown
in Figure 27. However, large source impedances significantly
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.
100
90
80
70
60
50
40
30
20
10
0
IN
1
is typically 70 Ω and is a lumped component comprised
Figure 29. Analog Input CMRR vs. Frequency
10
IN
and C
FREQUENCY (kHz)
PIN
IN
and the network formed by the
100
. C
PIN
is primarily the pin capac-
1000
PIN
.
IN
10000
is pri-