EVAL-AD7985FMCZ View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'EVAL-AD7985FMCZ' PDF : 28 Pages View PDF
Data Sheet
ANALOG INPUTS
Figure 24 shows an equivalent circuit of the input structure of
the AD7985.
The two diodes, D1 and D2, provide ESD protection for the
analog inputs, IN+ and IN−. Take care to ensure 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 protect the device.
REF
IN+ OR IN–
REFGND
D1
CPIN
D2
RIN
CIN
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+ and IN−) can be modeled as a parallel combination
of Capacitor CPIN and the network formed by the series connection
of Resistor RIN and Capacitor CIN. CPIN is primarily the pin
capacitance. RIN is typically 400 Ω and is a lumped component
composed of serial resistors and the on resistance of the switches.
CIN 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 CPIN. RIN and CIN 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 AD7985 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.
AD7985
DRIVER AMPLIFIER CHOICE
Although the AD7985 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 AD7985. The noise from the driver is
filtered by the AD7985 analog input circuit one-pole, low-
pass filter, made by RIN and CIN, or by the external filter, if
one is used. Because the typical noise of the AD7985 is
50 µV rms, the SNR degradation due to the amplifier is
SNR LOSS
= 20
log
50
502
+
π
2
f −3dB (NeN )2
where:
f–3dB is the input bandwidth, in megahertz, of the AD7985
(19 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 operational
amplifier in nV/√Hz.
• For ac applications, the driver must have a THD perfor-
mance commensurate with that of the AD7985.
• For multichannel multiplexed applications, the driver
amplifier and the AD7985 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 driver amplifier,
settling at 0.1% to 0.01% is more commonly specified. This
value may differ significantly from the settling time at a 16-bit
level and must 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. C | Page 15 of 28
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