EVAL-AD7693CB Analog Devices Inc, EVAL-AD7693CB Datasheet - Page 14

• Manufacturer Part Number: EVAL-AD7693CB
• Description: BOARD EVAL FOR AD7693 ADC
• Manufacturer: Analog Devices Inc
• Series: PulSAR®r

Specifications of EVAL-AD7693CB

• Number Of Adc's: 1
• Number Of Bits: 16
• Sampling Rate (per Second): 500k
• Data Interface: Serial
• Inputs Per Adc: 1 Differential
• Input Range: ±VREF
• Power (typ) @ Conditions: 18mW @ 500kSPS
• Voltage Supply Source: Single
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD7693
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

AD7693
ANALOG INPUTS
Figure 28 shows an equivalent circuit of the input structure of
the AD7693.
The two diodes, D1 and D2, provide ESD protection for the
analog inputs, IN+ and IN−. Care must be taken to ensure that
the analog input signal does not exceed 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 130 mA maximum. For instance,
these conditions could eventually occur when the input buffer’s
(U1) supplies are different from VDD. In such a case, for
example, an input buffer with a short circuit, the current
limitation can be used to protect the part.
The analog input structure allows the sampling of the true
differential signal between IN+ and IN−. By using these
differential inputs, signals common to both inputs are rejected.
During the acquisition phase, the impedance of the analog
inputs (IN+ and IN−) can be modeled as a parallel combination
of the capacitor, C
connection of R
R
serial resistors and the on resistance of the switches. C
typically 30 pF and is mainly the ADC sampling capacitor.
During the conversion phase, where the switches are opened,
the input impedance is limited to C
low-pass filter that reduces undesirable aliasing effects and
limits the noise.
When the source impedance of the driving circuit is low, the
AD7693 can be driven directly. Large source impedances
significantly affect the ac performance, especially total
IN
is typically 600 Ω and is a lumped component made up of
OR IN–
100
GND
95
90
85
80
75
70
65
60
55
50
IN+
1
Figure 29. Analog Input CMRR vs. Frequency
Figure 28. Equivalent Analog Input Circuit
IN
PIN
and C
C
PIN
10
, and the network formed by the series
VDD
IN
. C
FREQUENCY (kHz)
D1
D2
PIN
is primarily the pin capacitance.
100
PIN
. R
IN
and C
R
IN
1000
IN
VREF = 5V
make a 1-pole,
C
IN
IN
10000
is
Rev. 0 | Page 14 of 24
harmonic distortion (THD). The dc performances are less
sensitive to the input impedance. 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.
DRIVER AMPLIFIER CHOICE
Although the AD7693 is easy to drive, the driver amplifier must
meet the following requirements:
•
•
•
The noise generated by the driver amplifier needs to be
kept as low as possible to preserve the SNR and transition
noise performance of the AD7693. The noise coming from
the driver is filtered by the AD7693 analog input circuit’s
1-pole, low-pass filter made by R
external filter, if one is used. Because the typical noise of
the AD7693 is 56 μV rms, the SNR degradation due to the
amplifier is
where:
f
(9 MHz) or the cutoff frequency of the input filter, if
one is used.
N is the noise gain of the amplifier (for example, 1 in
buffer configuration).
e
in nV/√Hz.
For ac applications, the driver should have a THD
performance commensurate with the AD7693.
For multichannel multiplexed applications, the driver
amplifier and the AD7693 analog input circuit must settle
for a full-scale step onto the capacitor array at a 16-bit level
(0.0015%, 15 ppm). In the amplifier’s data sheet, settling at
0.1% to 0.01% is more commonly specified. This could
differ significantly from the settling time at a 16-bit level
and should be verified prior to driver selection.
Figure 30. THD vs. Analog Input Frequency and Source Resistance
SNR
−3 dB
–100
–105
–110
–115
–120
–125
–130
N
–80
–85
–90
–95
is the equivalent input noise voltage of the op amp,
B
LOSS
0
is the input bandwidth in megahertz of the AD7693
VDD = 5V
=
10
20
log
20
⎛
⎜
⎜
⎜
⎜ ⎜
⎝
30
56
100Ω
50Ω
33Ω
250Ω
FREQUENCY (kHz)
2
+
40
π
2
f
−
50
3
dB
IN
(
and C
Ne
60
56
N
)
2
IN
70
+
or by the
π
2
f
80
−
3
dB
(
90
Ne
N
)
2
⎞
⎟
⎟
⎟
⎟ ⎟
⎠