AD7985 Analog Devices, AD7985 Datasheet - Page 15

• Manufacturer Part Number: AD7985
• Description: 16-Bit, 2.5 MSPS PulSAR 11 mW ADC in QFN
• Manufacturer: Analog Devices

Specifications of AD7985

• Resolution (bits): 16bit
• # Chan: 1
• Sample Rate: 2.5MSPS
• Interface: Ser,SPI
• Analog Input Type: SE-Uni
• Ain Range: (Vref) p-p,Uni (Vref)
• Adc Architecture: SAR
• Pkg Type: CSP

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−. 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.
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 C
of R
400 Ω and is a lumped component composed of serial resistors
and the on resistance of the switches. C
is mainly the ADC sampling capacitor.
During the sampling phase, where the switches are closed, the
input impedance is limited to C
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.
IN
and C
IN+ OR IN–
REFGND
IN
PIN
. C
Figure 24. Equivalent Analog Input Circuit
and the network formed by the series connection
PIN
is primarily the pin capacitance. R
C
PIN
REF
D1
D2
PIN
. R
IN
and C
IN
is typically 30 pF and
R
IN
IN
make a one-pole,
C
IN
IN
is typically
Rev. A | Page 15 of 28
DRIVER AMPLIFIER CHOICE
Although the AD7985 is easy to drive, the driver amplifier must
meet the following requirements:
•
•
•
Table 8. Recommended Driver Amplifiers
Amplifier
AD8021
AD8022
ADA4899-1
AD8014
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’s one-pole, low-
pass filter, made by R
one is used. Because the typical noise of the AD7985 is
50 µV rms, the SNR degradation due to the amplifier is
where:
f
(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
nV/√Hz.
For ac applications, the driver should 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 should be verified prior to driver selection.
SNR
–3dB
N
is the equivalent input noise voltage of the op amp, in
is the input bandwidth, in megahertz, of the AD7985
LOSS
=
20
log
Typical Application
Very low noise and high frequency
Low noise and high frequency
Ultralow noise and high frequency
Low power and high frequency






50
IN
2
+
and C
π
2
50
f
−
3dB
IN
, or by the external filter, if
(
Ne
N
)
2






AD7985