AD7699
Data Sheet
Rev. B | Page 18 of 28
DRIVER AMPLIFIER CHOICE
Although the
AD7699 is easy to drive, the driver amplifier must
meet the following requirements:
The noise generated by the driver amplifier must be kept
as low as possible to preserve the SNR and transition noise
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 from the amplifier is
filtered by the
AD7699 analog input circuit low-pass filter
made by RIN and CIN or by an external filter, if one is used.
Because the typical noise of the
AD7699 is 35 μV rms (with
VREF = 5 V), the SNR degradation due to the amplifier is
2
)
(
2
π
35
log
20
N
3dB
LOSS
Ne
f
SNR
where:
f3dB
is the input bandwidth in megahertz of the AD7699
(14.7 MHz in full BW or 670 kHz in BW) or the cutoff
frequency of an input filter, if one is used.
N
is the noise gain of the amplifier (for example, 1 in buffer
configuration).
eN
is the equivalent input noise voltage of the op amp, in
nV/√Hz.
For ac applications, the driver should have a THD perfor-
THD vs. frequency for the
AD7699.
For multichannel, multiplexed applications on each input
or input pair, the driver amplifier and the
AD7699 analog
input circuit must settle a full-scale step onto the capacitor
array at a 16-bit level (0.0015%). In amplifier data sheets,
settling at 0.1% to 0.01% is more commonly specified. This
may differ significantly from the settling time at a 16-bit
level and should be verified prior to driver selection.
Table 7. Recommended Driver Amplifiers
Amplifier
Typical Application
Very low noise, small, and low power
5 V single supply, low noise
Very low noise and high frequency
Low noise and high frequency
Low power, low noise, and low frequency
5 V single supply, low power
VOLTAGE REFERENCE OUTPUT/INPUT
The
AD7699 allows the choice of a very low temperature drift
internal voltage reference, an external reference, or an external
buffered reference.
The internal reference of the
AD7699 provides excellent
performance and can be used in almost all applications. There
are five possible choices of voltage reference schemes briefly
described in
Table 8 with more details in each of the following
sections.
Internal Reference/Temperature Sensor
The internal reference can be set for a 4.096 V output as
detailed in
Table 8. With the internal reference enabled, the
band gap voltage is also present on the REFIN pin, which
requires an external 0.1 μF capacitor. Because the current
output of REFIN is limited, it can be used as a source if followed
by a suitable buffer, such as the
AD8605.
Enabling the reference also enables the internal temperature
sensor, which measures the internal temperature of the
AD7699and is thus useful for performing a system calibration. For
applications requiring the use of the temperature sensor, the
internal reference must be active (internal buffer can be
disabled in this case). Note that, when using the temperature
sensor, the output is straight binary referenced from the
The internal reference is temperature-compensated to within
15 mV. The reference is trimmed to provide a typical drift of
3 ppm/°C.
External Reference and Internal Buffer
For improved drift performance, an external reference can be
used with the internal buffer. The external reference is connected
to REFIN, and the output is produced on the REF pin. An
external reference can be used with the internal buffer with or
without the temperature sensor enabled. Refer to
Table 8 for
register details. With the buffer enabled, the gain is unity and is
limited to an input/output of 4.096 V.
The internal reference buffer is useful in multiconverter applica-
tions because a buffer is typically required in these applications. In
addition, a low power reference can be used because the internal
buffer provides the necessary performance to drive the SAR
External Reference
In any of the five voltage reference schemes, an external refer-
ence can be connected directly on the REF pin because the output
impedance of REF is >5 kΩ. To reduce power consumption, the
reference and buffer can be powered down independently or
together for the lowest power consumption. When only using the
external reference (and optional reference buffer as shown in
register details. For improved drift performance, an external