AD7980 Analog Devices, AD7980 Datasheet - Page 15

• Manufacturer Part Number: AD7980
• Manufacturer: Analog Devices

Specifications of AD7980

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

ANALOG INPUT
Figure 28 shows an equivalent circuit of the input structure of
the AD7980.
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 input buffer’s
(U1) 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 some serial
resistors and the on resistance of the switches. C
30 pF and is mainly the ADC sampling capacitor. During the
conversion phase, where the switches are opened, the input
impedance is limited to C
filter that reduces undesirable aliasing effects and limits the noise.
When the source impedance of the driving circuit is low, the
AD7980 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
OR IN–
GND
IN+
IN
PIN
. C
, and the network formed by the series connection of
PIN
Figure 28. Equivalent Analog Input Circuit
C
is primarily the pin capacitance. R
PIN
REF
PIN
D1
D2
. R
IN
and C
IN
make a 1-pole, low-pass
R
IN
C
IN
IN
is typically
IN
is typically
Rev. B | Page 15 of 28
DRIVER AMPLIFIER CHOICE
Although the AD7980 is easy to drive, the driver amplifier
needs to meet the following requirements:



Table 8. Recommended Driver Amplifiers
Amplifier
ADA4841
AD8021
AD8022
OP184
AD8655
AD8605,
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 AD7980. The noise coming from
the driver is filtered by the AD7980 analog input circuit’s
1-pole, low-pass filter made by R
filter, if one is used. Because the typical noise of the AD7980 is
47.3 μV rms, the SNR degradation due to 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 AD7980.
For multichannel multiplexed applications, the driver
amplifier and the AD7980 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 amplifier’s data sheet, 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.
–3dB
N
is the equivalent input noise voltage of the op amp,
AD8615
is the input bandwidth in MHz of the AD7980
SNR
LOSS
Typical Application
Very low noise, small 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
log






47.3
IN
2
and C

π
2
47.3
f

IN
3dB
or by the external
(
Ne
N
AD7980
)
2





