AD7986BCPZ Datasheet, PDF (15/28 Pages) Analog Devices – 18-Bit, 2 MSPS PulSAR 15 mW ADC in LFCSP (QFN)

ANALOG INPUTS
Figure 24 shows an equivalent circuit of the input structure of
the AD7986.
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 reference input
voltage (REF) by more than 0.3 V. If the analog input signal
exceeds this level, the diodes become forward-biased and start
conducting current. These diodes can handle a forward-biased
current of 130 mA maximum. However, if the supplies of the
input buffer (for example, the V+ and V− supplies of the buffer
amplifier in Figure 23) are different from those of REF, the
analog input signal may eventually exceed the supply rails by
more than 0.3 V. 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
C_PIN
D2
R_IN
C_IN
Figure 24. 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+ or IN−) can be modeled as a parallel combination
of Capacitor C_PIN and the network formed by the series connection
of R_IN and C_IN. C_PIN is primarily the pin capacitance. R_IN is typically
400 Ω and is a lumped component composed of serial resistors
and the on resistance of the switches. C_IN is typically 30 pF and
is mainly the ADC sampling capacitor.
During the sampling phase, where the switches are closed, the
input impedance is limited to C_PIN. R_IN and C_IN make a one-pole,
low-pass filter that reduces undesirable aliasing effects and
limits noise.
When the source impedance of the driving circuit is low, the
AD7986 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.
AD7986
DRIVER AMPLIFIER CHOICE
Although the AD7986 is easy to drive, the driver amplifier must
meet the following requirements:
• The noise generated by the driver amplifier must be kept as
low as possible to preserve the SNR and transition noise
performance of the AD7986. The noise from the driver is
filtered by the AD7986 analog input circuit’s one-pole, low-
pass filter, made by R_IN and C_IN or by the external filter, if
one is used. Because the typical noise of the AD7986 is
62.5 µV rms, the SNR degradation due to the amplifier is

SNR _LOSS
=
20
log





62.5

34.4 ²
+^π
2
f _−3dB (Ne_N )²



where:
f_–3dB is the input bandwidth, in megahertz, of the AD7986
(20 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_N is the equivalent input noise voltage of the op amp, in
nV/√Hz.
• For ac applications, the driver should have a THD perfor-
mance commensurate with the AD7986.
• For multichannel multiplexed applications, the driver
amplifier and the AD7986 analog input circuit must settle
for a full-scale step onto the capacitor array at an 18-bit level
(0.0004%, 4 ppm). In the data sheet of the driver amplifier,
settling at 0.1% to 0.01% is more commonly specified. This
may differ significantly from the settling time at an 18-bit
level and should be verified prior to driver selection.
Table 8. Recommended Driver Amplifiers
Amplifier
Typical Application
AD8021
Very low noise and high frequency
AD8022
Low noise and high frequency
ADA4899-1
Ultralow noise and high frequency
AD8014
Low power and high frequency
Rev. B | Page 15 of 28