AD7634 Analog Devices, AD7634 Datasheet - Page 20

• Manufacturer Part Number: AD7634
• Manufacturer: Analog Devices

Specifications of AD7634

• Resolution (bits): 18bit
• # Chan: 1
• Sample Rate: 670kSPS
• Interface: Byte,Par,Ser,SPI
• Analog Input Type: Diff-Bip,Diff-Uni
• Ain Range: 10V p-p,20 V p-p,40 V p-p
• Adc Architecture: SAR
• Pkg Type: CSP,QFP

AD7634
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 V range)
inputs. See Table 6 for pin details; see the Hardware Configuration
section and the Software Configuration section for program-
ming 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 AD7634.
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 of the AD7634 is a true differential
structure allowing 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. AD7634 Simplified Analog Input
C
PIN
VCC
D1
D2
RANGE ONLY
0V TO 5V
AVDD
D3
D4
R
AGND
IN
C
IN
Rev. A | Page 20 of 32
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
the series connection of R
capacitance. R
comprised of serial resistors and the on resistance of the switches.
C
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
Because the input impedance of the AD7634 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
AD7634 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 imped-
ances significantly affect the ac performance, especially the
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, as
shown in Figure 30.
IN
is primarily the ADC sampling capacitor and depending on
Figure 30. THD vs. Analog Input Frequency and Source Resistance
–110
–130
120
100
–70
–90
80
60
40
20
0
1
0
0V TO 5V
Figure 29. Analog Input CMRR vs. Frequency
IN
is typically 70 Ω and is a lumped component
±10V
10
25
200Ω
100Ω
33Ω
0V TO 10V
15Ω
FREQUENCY (kHz)
IN
FREQUENCY (kHz)
PIN
and C
and the network formed by
100
50
IN
. C
PIN
±5V
is primarily the pin
1000
75
10000
PIN
100
.