EVAL-AD7686CBZ Analog Devices Inc, EVAL-AD7686CBZ Datasheet - Page 15

• Manufacturer Part Number: EVAL-AD7686CBZ
• Description: BOARD EVALUATION FOR AD7686
• Manufacturer: Analog Devices Inc
• Series: PulSAR®r

Specifications of EVAL-AD7686CBZ

• 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 Supply
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD7686
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

DRIVER AMPLIFIER CHOICE
Although the AD7686 is easy to drive, the driver amplifier
should meet the following requirements:
•
•
•
Table 8. Recommended Driver Amplifiers
Amplifier
ADA4841-x
AD8605, AD8615
AD8655
OP184
AD8021
AD8022
AD8519
AD8031
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 AD7686. Note that the AD7686
has a noise much lower than most of the other 16-bit
ADCs and, therefore, can be driven by a noisier amplifier
to meet a given system noise specification. The noise
coming from the amplifier is filtered by the AD7686 analog
input circuit 1-pole, low-pass filter made by R
by the external filter, if one is used. Because the typical
noise of the AD7686 is 37 μV rms, the SNR degradation
due to the amplifier is
where:
f
(9 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 AD7686. Figure 18
shows the THD vs. frequency that the driver should exceed.
For multichannel multiplexed applications, the driver
amplifier and the AD7686 analog input circuit must settle a
full-scale step onto the capacitor array at a 16-bit level
(0.0015%). In the data sheet for the amplifier, 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,
is the input bandwidth in MHz of the AD7686
SNR
LOSS
=
Typical Application
Very low noise and low power
5 V single-supply, low power
5 V single-supply, low power
Low power, low noise, and low frequency
Very low noise and high frequency
Very low noise and high frequency
Small, low power and low frequency
High frequency and low power
20log
⎛
⎜
⎜
⎜
⎜
⎝
37
2
+
π
2
37
f
−
3dB
(
Ne
N
)
2
IN
⎞
⎟
⎟
⎟
⎟
⎠
and C
IN
Rev. B | Page 15 of 28
or
VOLTAGE REFERENCE INPUT
The AD7686 voltage reference input, REF, has a dynamic input
impedance and should, therefore, be driven by a low impedance
source with efficient decoupling between the REF and GND
pins, as explained in the Layout section.
When REF is driven by a very low impedance source, such as a
reference buffer using the
(X5R, 0805 size) ceramic chip capacitor is appropriate for
optimum performance.
If an unbuffered reference voltage is used, the decoupling value
depends on the reference used. For instance, a 22 μF (X5R,
1206 size) ceramic chip capacitor is appropriate for optimum
performance using a low temperature drift
If desired, smaller reference decoupling capacitor values down
to 2.2 μF can be used with a minimal impact on performance,
especially DNL.
Regardless, there is no need for an additional lower value
ceramic decoupling capacitor, such as 100 nF, between the REF
and GND pins.
POWER SUPPLY
The AD7686 is specified at 4.5 V to 5.5 V. The device uses two
power supply pins: a core supply VDD and a digital input/
output interface supply VIO. VIO allows direct interface with
any logic between 1.8 V and VDD. To reduce the supplies
needed, the VIO and VDD can be tied together.
The AD7686 is independent of power supply sequencing
between VIO and VDD. Additionally, it is very insensitive to
power supply variations over a wide frequency range, as shown
in Figure 30, which represents PSRR over frequency.
100
110
90
80
70
60
50
40
30
1
Figure 30. PSRR vs. Frequency
10
AD8031
FREQUENCY (kHz)
100
or the AD8605, a 10 μF
VDD = 5V
ADR43x
1000
AD7686
reference.
10000