EVAL-CONTROLBOARD AD [Analog Devices], EVAL-CONTROLBOARD Datasheet - Page 12

• Manufacturer Part Number: EVAL-CONTROLBOARD
• Description: 16-Bit, 100 kSPS CMOS ADC
• Manufacturer: AD [Analog Devices]

TYPICAL CONNECTION DIAGRAM
Figure 6 shows a typical connection diagram for the AD7660.
Analog Input
Figure 5 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 for-
ward-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
case, 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 CMR 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.
AD7660
OR INGND
AGND
ANALOG INPUT
IN
(0V TO 2.5V)
ANALOG
SUPPLY
(5V)
2.5V REF
C1
1
AVDD
D1
D2
C
REF
U1
1
10 F
2
NOTES:
1
2
3
WITH THE AD780 OR THE ADR291 VOLTAGE REFERENCE, C
THE AD8519 IS RECOMMENDED
OPTIONAL LOW JITTER CNVST
100nF
100nF
R1
REF
REFGND
IN
INGND
AVDD
PD
C2
AGND
100
RESET
10 F
AD7660
DGND
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 Ω and is a lumped component made up of some
serial resistor 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 that reduces undesirable
aliasing effect and limits the noise.
100nF
DVDD
80
70
60
50
40
30
20
10
0
0.1k
OVDD
REF
IS 47 F
COMMON-MODE INPUT FREQUENCY – Hz
SER/PAR
1k
SDOUT
CNVST
OGND
OB/2C
SCLK
BUSY
CS
RD
100nF
10k
DVDD
D
10 F
3
SERIAL
PORT
100k
CLOCK
DIGITAL SUPPLY
(3.3V OR 5V)
C/ P/DSP
1M
10M