AD7988-1 Analog Devices, AD7988-1 Datasheet - Page 16

• Manufacturer Part Number: AD7988-1
• Description: 16-Bit, 100ksps, Ultra Low Power 16-Bit SAR ADC
• Manufacturer: Analog Devices

Specifications of AD7988-1

• Resolution (bits): 16bit
• # Chan: 1
• Sample Rate: 100kSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: (Vref) p-p,5V p-p,Uni (Vref),Uni 5.0V
• Pkg Type: SOP

AD7988-1/AD7988-5
ANALOG INPUTS
Figure 32 shows an equivalent circuit of the input structure of
the AD7988-x.
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 may eventually occur when the input buffer’s
supplies 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.
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
R
400 Ω and is a lumped component made up of serial resistors
and the on resistance of the switches. C
is mainly the ADC sampling capacitor. During the conversion
phase, when the switches are opened, the input impedance is
limited to C
reduces undesirable aliasing effects and limits the noise.
When the source impedance of the driving circuit is low, the
AD7988-x
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
OR IN–
GND
IN+
IN
can be driven directly. Large source impedances
PIN
. C
PIN
. R
PIN
and the network formed by the series connection of
Figure 32. Equivalent Analog Input Circuit
IN
is primarily the pin capacitance. R
C
and C
PIN
REF
IN
D1
D2
make a one-pole, low-pass filter that
IN
R
IN
is typically 30 pF and
C
IN
IN
is typically
Rev. A | Page 16 of 24
DRIVER AMPLIFIER CHOICE
Although the
needs to meet the following requirements:
•
•
•
Table 8. Recommended Driver Amplifiers
Amplifier
ADA4841-1
AD8021
AD8022
OP184
AD8655
AD8605,
The noise generated by the driver amplifier must be kept as
low as possible to preserve the SNR and transition noise
performance of the AD7988-x. The noise coming from the
driver is filtered by the
one-pole, low-pass filter made by R
external filter, if one is used. Because the typical noise of
the
the amplifier is
where:
f
(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).
e
in nV/√Hz.
For ac applications, the driver should have a THD
performance commensurate with the AD7988-x.
For multichannel multiplexed applications, the driver ampli-
fier and the
a full-scale step onto the capacitor array at a 16-bit level
(0.0015%, 15 ppm). In the amplifier data sheet, settling at
0.1% to 0.01% is more commonly specified. This may
differ significantly from the settling time at a 16-bit level
and should be verified prior to driver selection.
–3dB
N
is the equivalent input noise voltage of the op amp,
AD7988-x
AD8615
is the input bandwidth in MHz of the
SNR
AD7988-x
AD7988-x
LOSS
is 47.3 µV rms, the SNR degradation due to
Typical Application
Very low noise, small size, and low power
Very low noise and high frequency
Low noise and high frequency
Low power, low noise, and low frequency
5 V single-supply, low noise
5 V single-supply, low power
=
20
is easy to drive, the driver amplifier
log
analog input circuit must settle for
AD7988-x






47.3
2
analog input circuit’s
+
IN
π
2
47.3
and C
f
−
3dB
Data Sheet
AD7988-x
(
IN
Ne
or by the
N
)
2





