AD7983 View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'AD7983' PDF : 24 Pages View PDF
Data Sheet
ANALOG INPUTS
Figure 23 shows an equivalent circuit of the input structure of
the AD7983.
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 become forward-
biased 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 supplies of
the input buffer (U1) 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.
REF
IN+
OR IN–
GND
D1
CPIN
D2
RIN
CIN
Figure 23. Equivalent Analog Input Circuit
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
capacitor, CPIN, and the network formed by the series connection of
RIN and CIN. CPIN is primarily the pin capacitance. RIN is typically
400 Ω and is a lumped component made up of some serial
resistors and the on resistance of the switches. CIN is 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 CPIN. RIN and CIN make a 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
AD7983 can be driven directly. Large source impedances
significantly affect the ac performance, especially 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.
AD7983
DRIVER AMPLIFIER CHOICE
Although the AD7983 is easy to drive, the driver amplifier
needs to 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 AD7983. The noise coming from
the driver is filtered by the AD7983 analog input circuit’s
1-pole, low-pass filter made by RIN and CIN or by the external
filter, if one is used. Because the typical noise of the AD7983
is 39.7 µV rms, the SNR degradation due to the amplifier is
SNR LOSS
=
20
log
39.7
39.72
+
π
2
f −3dB (NeN )2
where:
f–3dB is the input bandwidth in MHz of the AD7983
(10 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).
eN is the equivalent input noise voltage of the op amp,
in nV/√Hz.
• For ac applications, the driver should have a THD
performance commensurate with the AD7983.
• For multichannel multiplexed applications, the driver
amplifier and the AD7983 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 data sheet of the amplifier, 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.
The Precision ADC Driver Tool can be used to model the
settling behavior and to estimate the ac performance of the
AD7983 with a selected driver and RC filter.
Table 10. Recommended Driver Amplifiers
Amplifier
Typical Application
ADA4805-1 Low noise, small size, and low power
ADA4807-1 Very low noise and high frequency
ADA4627-1 Precision, low noise, and low input bias current
ADA4522-1 Precision, zero drift, and EMI enhanced
ADA4841-1 Low noise, low distortion, and low power
Rev. C | Page 15 of 25
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