AD7664 Analog Devices, AD7664 Datasheet - Page 13

• Manufacturer Part Number: AD7664
• Manufacturer: Analog Devices

Specifications of AD7664

• Resolution (bits): 16bit
• # Chan: 1
• Sample Rate: 570kSPS
• Interface: Par,Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: (Vref) p-p,Uni (Vref)
• Adc Architecture: SAR
• Pkg Type: CSP,QFP

Analog Input
Figure 6 shows an equivalent circuit of the input structure of
the AD7664.
The two diodes D1 and D2 provide ESD protection for the
analog inputs IN and INGND. Care must be taken to ensure
that the analog input signal never exceeds the supply rails by more
than 0.3 V. This will cause these diodes to become forward-
biased and start conducting current. These diodes can handle
a forward-biased current of 100 mA maximum. For instance,
these conditions could eventually occur when the input buffer’s
(U1) supplies are different from AVDD. In such cases, an input
buffer with a short circuit current limitation can be used to
protect the part.
This analog input structure allows the sampling of the differen-
tial signal between IN and INGND. Unlike other converters,
the INGND input is sampled at the same time as the IN input.
By using this differential input, small signals common to both
inputs are rejected, as shown in Figure 7, which represents the
typical CMRR over frequency. For instance, by using INGND
to sense a remote signal ground, difference of ground potentials
between the sensor and the local ADC ground are eliminated.
During the acquisition phase, the impedance of the analog input
IN can be modeled as a parallel combination of capacitor C1
and the network formed by the series connection of R1 and C2.
Capacitor C1 is primarily the pin capacitance. The resistor R1 is
typically 140 Ω and is a lumped component made up of some
serial resistors and the on resistance of the switches. The capacitor
C2 is typically 60 pF and is mainly the ADC sampling capacitor.
During the conversion phase, where the switches are opened, the input
impedance is limited to C1. The R1, C2 makes a one-pole low-pass
filter that reduces the undesirable aliasing effect and limits the noise.
When the source impedance of the driving circuit is low, the
AD7664 can be driven directly. Large source impedances will
REV. E
OR INGND
Figure 7. Analog Input CMRR vs. Frequency
AGND
70
60
50
40
30
20
10
Figure 6. Equivalent Analog Input Circuit
0
1k
IN
C1
10k
AVDD
FREQUENCY – Hz
D1
D2
100k
R1
C2
1M
–13–
significantly affect the ac performances, especially the total
harmonic distortion (THD). The maximum source impedance
depends on the amount of THD that can be tolerated. The
THD degrades in function of the source impedance and the
maximum input frequency as shown in Figure 8.
Driver Amplifier Choice
Although the AD7664 is easy to drive, the driver amplifier needs
to meet at least the following requirements:
where:
• The driver amplifier and the AD7664 analog input circuit
• The noise generated by the driver amplifier needs to be kept as
f
N
e
–3 dB
N
must be able, together, to settle for a full-scale step of the
capacitor array at a 16-bit level (0.0015%). In the amplifier’s
data sheet, the settling at 0.1% to 0.01% is more commonly
specified. It could significantly differ from the settling time at
16-bit level and it should, therefore, be verified prior to the
driver selection. The tiny op amp AD8021, which combines
ultralow noise and a high gain bandwidth, meets this settling
time requirement even when used with high gain up to 13.
low as possible in order to preserve the SNR and transition
noise performance of the AD7664. The noise coming from
the driver is filtered by the AD7664 analog input circuit one-
pole low-pass filter made by R1 and C2 or the external filter, if
any is used. The SNR degradation due to the amplifier is:
Figure 8. THD vs. Analog Input Frequency and
Source Resistance
–100
–70
–75
–80
–85
–90
–95
is the –3 dB input bandwidth in MHz of the AD7664
(18 MHz) or the cutoff frequency of the input filter, if
any used.
is the noise gain of the amplifier (1, if in buffer
configuration).
is the equivalent input noise voltage of the op amp in
nV/√Hz.
SNR
10
LOSS
=
R = 100
20
log
FREQUENCY – kHz






784
100
+
π
2
28
f
–3dB
R = 11
(
Ne
R = 50
AD7664
N
)
2






1000