ad7688brm Analog Devices, Inc., ad7688brm Datasheet - Page 14

• Manufacturer Part Number: ad7688brm
• Description: 16-bit, 1.5 Lsb Inl, 500 Ksps Pulsar? Differential Adc In Msop/qfn
• Manufacturer: Analog Devices, Inc.

AD7688
ANALOG INPUT
Figure 27 shows an equivalent circuit of the input structure of
the AD7688.
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 never exceeds the supply rails by more
than 0.3 V because this causes these diodes to begin to forward-
bias and 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, an input
buffer with a short-circuit 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,
as shown in Figure 28, which represents the typical CMRR over
frequency.
OR IN–
GND
IN+
80
70
60
1
Figure 28. Analog Input CMRR vs. Frequency
Figure 27. Equivalent Analog Input Circuit
C
PIN
10
VDD
VDD = 5V
D1
D2
FREQUENCY (kHz)
100
R
IN
1000
C
IN
10000
Rev 0 | Page 14 of 28
During the acquisition phase, the impedance of the analog
inputs (IN+ or IN−) can be modeled as a parallel combination
of capacitor, C
connection of R
R
some 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
1-pole, low-pass filter that reduces undesirable aliasing effects
and limits the noise.
When the source impedance of the driving circuit is low, the
AD7688 can be driven directly. Large source impedances
significantly affect the ac performance, especially total
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, as shown in
Figure 29.
IN
–100
–110
–120
is typically 600 Ω and is a lumped component made up of
–60
–70
–80
–90
Figure 29. THD vs. Analog Input Frequency and Source Resistance
0
R
R
R
R
S
S
S
S
= 250Ω
= 100Ω
= 50Ω
= 33Ω
PIN
IN
, and the network formed by the series
and C
25
IN
. C
FREQUENCY (kHz)
PIN
is primarily the pin capacitance.
50
PIN
. R
IN
and C
75
IN
make a
IN
100
is