EVAL-AD7686CB Analog Devices Inc, EVAL-AD7686CB Datasheet - Page 14

• Manufacturer Part Number: EVAL-AD7686CB
• Description: BOARD EVALUATION FOR AD7686
• Manufacturer: Analog Devices Inc
• Series: PulSAR®r

Specifications of EVAL-AD7686CB

• Number Of Adc's: 1
• Number Of Bits: 16
• Sampling Rate (per Second): 500k
• Data Interface: Serial
• Inputs Per Adc: 1 Differential
• Input Range: ±VREF
• Power (typ) @ Conditions: 15mW @ 500kSPS, 5 V
• Voltage Supply Source: Single
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD7686
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

AD7686
ANALOG INPUT
Figure 27 shows an equivalent circuit of the input structure
of the AD7686. 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 begin to forward-bias 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, an input buffer with a short-circuit
current limitation can be used to protect the part.
The analog input structure allows the sampling of the
differential signal between IN+ and IN−. By using this
differential input, small signals common to both inputs are
rejected, as shown in Figure 28, which represents the typical
CMRR over frequency. For instance, by using IN− to sense a
remote signal ground, ground potential differences between
the sensor and the local ADC ground are eliminated.
OR IN–
GND
IN+
80
70
60
50
40
1
Figure 28. Analog Input CMRR vs. Frequency
Figure 27. Equivalent Analog Input Circuit
C
PIN
10
VDD = 5V
VDD
D1
D2
FREQUENCY (kHz)
100
R
IN
1000
C
IN
10000
Rev. B | Page 14 of 28
During the acquisition phase, the impedance of the analog
inputs (IN+ or IN−) can be modeled as a parallel combination
of capacitor, C
connection of R
R
some serial resistors and the on resistance of the switches. C
typically 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
1-pole, low-pass filter that reduces undesirable aliasing effects
and limits the noise.
When the source impedance of the driving circuit is low, the
AD7686 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, as shown
in Figure 29.
IN
is typically 600 Ω and is a lumped component made up of
Figure 29. THD vs. Analog Input Frequency and Source Resistance
–100
–105
–110
–80
–85
–90
–95
0
R
R
R
R
S
S
S
S
PIN
= 100Ω
= 250Ω
= 50Ω
= 33Ω
IN
, and the network formed by the series
and C
25
IN
. C
FREQUENCY (kHz)
PIN
is primarily the pin capacitance.
50
PIN
. R
IN
and C
75
IN
make a
100
IN
is