AD7660ACP Analog Devices Inc, AD7660ACP Datasheet - Page 12

• Manufacturer Part Number: AD7660ACP
• Description: IC ADC 16BIT UNIPOLAR 48-LFCSP
• Manufacturer: Analog Devices Inc
• Series: PulSAR®r

Specifications of AD7660ACP

• Rohs Status: RoHS non-compliant
• Number Of Bits: 16
• Sampling Rate (per Second): 100k
• Data Interface: Serial, Parallel
• Number Of Converters: 1
• Power Dissipation (max): 25mW
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 48-VFQFN, CSP Exposed Pad
• For Use With: EVAL-AD7660CBZ - BOARD EVALUATION FOR AD7660

AD7660
Analog Input
Figure 6 shows an equivalent circuit of the input structure of the
AD7660.
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 differential
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 is 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 3242 W 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. It has to be noted that the input
impedance of the AD7660, unlike other SAR ADCs, is not a pure
capacitance and thus, inherently reduces the kickback transient at
the beginning of the acquisition phase. The R1, C2 makes a one-
pole low-pass filter with a typical cutoff frequency of 820 kHz that
reduces undesirable aliasing effect and limits the noise.
OR INGND
Figure 7. Analog Input CMRR vs. Frequency
85
80
75
70
65
60
55
50
45
40
AGND
Figure 6. Equivalent Analog Input Circuit
1k
IN
10k
C1
AVDD
FREQUENCY – Hz
D1
D2
100k
R1
1M
C2
10M
–12–
When the source impedance of the driving circuit is low, the
AD7660 can be driven directly. Large source impedances will
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 AD7660 is easy to drive, the driver amplifier needs
to meet at least the following requirements:
The SNR degradation due to the amplifier is:
where:
f
(0.82 MHz) or the cutoff frequency of the input filter if any
are used.
N is the noise factor of the amplifier (1 if in buffer configuration).
e
nV/÷Hz.
–3 dB
N
•
•
•
is the equivalent input noise voltage of the op amp in
The driver amplifier and the AD7660 analog input circuit
must be able, together, to settle for a full-scale step of
the capacitor array at a 16-bit level (0.0015%). For
instance, operation at the maximum throughput of 100 kSPS
requires a minimum gain bandwidth product of 5 MHz.
The noise generated by the driver amplifier needs to be
kept as low as possible in order to preserve the SNR and
transition noise performance of the AD7660. The noise
coming from the driver is filtered by the AD7660 analog
input circuit one-pole low-pass filter made by R1 and C2.
For instance, a driver with an equivalent input noise of
4 nV/÷Hz like the OP184 and configured as a buffer, thus
with a noise gain of +1, degrades the SNR by only 0.1 dB.
The driver needs to have a THD performance suitable to
that of the AD7660. TPC 8 gives the THD versus frequency
that the driver should preferably exceed.
is the –3 dB input bandwidth in MHz of the AD7660
–100
Figure 8. THD vs. Analog Input Frequency and
Source Resistance
–70
–75
–80
–85
–90
–95
SNR
1
LOSS
=
20
log
INPUT FREQUENCY – kHz
Ê
Á
Á
Á
Á
Ë
784
10
+
p
2
f
28
–
3
dB
(
N e
R
R
S
S
R
R
S
= 500
= 100
S
N
= 20
= 50
)
2
ˆ
˜
˜
˜
˜
¯
100
REV. D