ad7688brm Analog Devices, Inc., ad7688brm Datasheet - Page 15

• Manufacturer Part Number: ad7688brm
• Description: 16-bit, 1.5 Lsb Inl, 500 Ksps Pulsar? Differential Adc In Msop/qfn
• Manufacturer: Analog Devices, Inc.

DRIVER AMPLIFIER CHOICE
Although the AD7688 is easy to drive, the driver amplifier
needs to meet the following requirements:
•
•
•
Table 8. Recommended Driver Amplifiers
Amplifier
AD8021
AD8022
OP184
AD8605,
AD8519
AD8031
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 AD7688. Note that the
AD7688 has a noise much lower than most of the other
16-bit ADCs and, therefore, can be driven by a noisier op
amp while preserving the same or better system perform-
ance. The noise coming from the driver is filtered by the
AD7688 analog input circuit 1-pole, low-pass filter made
by R
Because the typical noise of the AD7688 is 53 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 AD7688. Figure 18
shows the THD vs. frequency that the driver should
exceed.
For multichannel multiplexed applications, the driver
amplifier and the AD7688 analog input circuit must settle
for a full-scale step onto the capacitor array at a 16-bit level
(0.0015%, 15 ppm). In the amplifier’s data sheet, 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 AD7688
AD8615
IN
and C
SNR
LOSS
IN
or by the external filter, if one is used.
=
Typical Application
Very low noise and high frequency
Low noise and high frequency
Low power, low noise, and low frequency
5 V single-supply, low power
Small, low power and low frequency
High frequency and low power
20log
⎛
⎜
⎜
⎜
⎜ ⎜
⎝
5
3
2
+
π
2
f
53
−
3dB
(
Ne
N
)
2
⎞
⎟
⎟
⎟
⎟ ⎟
⎠
Rev 0 | Page 15 of 28
SINGLE-TO-DIFFERENTIAL DRIVER
For applications using a single-ended analog signal, either
bipolar or unipolar, a single-ended-to-differential driver
allows for a differential input into the part. The schematic is
shown in Figure 30. When provided a single-ended input signal,
this configuration produces a differential ±V
at V
VOLTAGE REFERENCE INPUT
The AD7688 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, for
example, a reference buffer using the AD8031 or the AD8605, a
10 µF (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 ADR43x reference.
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 (for example, 100 nF) between the
REF and GND pins.
POWER SUPPLY
The AD7688 is specified at 4.5 V to 5.5 V. It has, unlike other
low voltage converters, a low enough noise to design a 16-bit
resolution system with low supply and respectable performance.
It 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 AD7688 is
independent of power supply sequencing between VIO and
VDD. Additionally, it is very insensitive to power supply
ANALOG INPUT
(±10V, ±5V, ..)
REF
/2.
VREF
Figure 30. Single-Ended-to-Differential Driver Circuit
VREF
10kΩ
10kΩ
100nF
590Ω
U1
100nF
590Ω
590Ω
U2
VREF
REF
10µF
with midscale
IN+
IN–
AD7688
REF
AD7688