EVAL-AD7689EDZ Datasheet, PDF (21/32 Pages) Analog Devices – 16-Bit, 4-Channel/8-Channel, 250 kSPS PulSAR ADC

Data Sheet
DRIVER AMPLIFIER CHOICE
Although the AD7682/AD7689 are 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
performance of the AD7682/AD7689. Note that the AD7682/
AD7689have 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 from the amplifier is filtered by the AD7682/AD7689
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 AD7682/AD7689 is 35 μV rms (with VREF = 5 V), the
SNR degradation due to the amplifier is
⎜⎛
SNR _LOSS
= 20 log
⎜
⎜
⎜⎜⎝
⎟⎞
35
⎟
35²
+
π
2
f _−3dB (Ne _N )²
⎟
⎟⎟⎠
where:
f–3dB is the input bandwidth in megahertz of the AD7682/
AD7689 (1.7 MHz in full BW or 425 kHz in ¼ 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-
mance commensurate with the AD7682/AD7689. Figure 18
shows THD vs. frequency for the AD7682/AD7689.
• For multichannel, multiplexed applications on each input
or input pair, the driver amplifier and the AD7682/AD7689
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 9. Recommended Driver Amplifiers
Amplifier
Typical Application
ADA4841-x
Very low noise, small, and low power
AD8655
5 V single supply, low noise
AD8021
Very low noise and high frequency
AD8022
Low noise and high frequency
OP184
Low power, low noise, and low frequency
AD8605, AD8615 5 V single supply, low power
AD7682/AD7689
VOLTAGE REFERENCE OUTPUT/INPUT
The AD7682/AD7689 allow the choice of a very low temper-
ature drift internal voltage reference, an external reference, or an
external buffered reference.
The internal reference of the AD7682/AD7689 provide excel-
lent performance and can be used in almost all applications.
There are six possible choices of voltage reference schemes
briefly described in Table 10, with more details in each of the
following sections.
Internal Reference/Temperature Sensor
The precision internal reference, suitable for most applications,
can be set for either a 2.5 V or a 4.096 V output, as detailed in
Table 10. 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. Note that the voltage of REFIN
changes depending on the 2.5 V or 4.096 V internal reference.
Enabling the reference also enables the internal temperature sensor,
which measures the internal temperature of the AD7682/AD7689,
and 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 AD7682/AD7689 GND pin.
The internal reference is temperature-compensated to within
10 mV. The reference is trimmed to provide a typical drift of
±10 ppm/°C.
Connect the AD7682/AD7689 as shown in Figure 31 for either
a 2.5 V or 4.096 V internal reference.
10µF
100nF
REF REFIN
AD7682/
AD7689 TEMP
GND
Figure 31. 2.5 V or 4.096 V Internal Reference Connection
Rev. D | Page 21 of 32