Datasheet

Data Sheet AD7949
Rev. D | Page 19 of 32
DRIVER AMPLIFIER CHOICE
Although the AD7949 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
performance of the AD7949. Note that the AD7949 has a
noise much lower than most of the other 14-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 AD7949 analog input circuit
low-pass filter made by R
IN
and C
IN
or by an external filter,
if one is used.
 For ac applications, the driver should have a THD perfor-
mance commensurate with the AD7949. Figure 17 shows
THD vs. frequency for the AD7949.
 For multichannel, multiplexed applications on each input
or input pair, the driver amplifier and the AD7949 analog
input circuit must settle a full-scale step onto the capacitor
array at a 14-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 14-bit
level and should be verified prior to driver selection.
Table 8. 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
VOLTAGE REFERENCE OUTPUT/INPUT
The AD7949 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 AD7949 provides excellent
performance and can be used in almost all applications. There
are six possible choices of voltage reference schemes briefly
described in Table 9, 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 9. With the internal reference enabled, the band gap
voltage is also present on the REFIN pin, which requires an
external 0.1 F 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 AD7949
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
AD7949 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 AD7949 as shown in Figure 30 for either a 2.5 V or
4.096 V internal reference.
REF
GND
TEMP
AD7949
10µF
100nF
REFIN
0
7351-030
Figure 30. 2.5 V or 4.096 V Internal Reference Connection