AD7949 Analog Devices, AD7949 Datasheet - Page 18

• Manufacturer Part Number: AD7949
• Description: 14-Bit, 8-Channel, 250 kSPS PulSAR ADC
• Manufacturer: Analog Devices

Specifications of AD7949

• Resolution (bits): 14bit
• # Chan: 8
• Sample Rate: 250kSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni,SE-Uni
• Adc Architecture: SAR
• Pkg Type: CSP

AD7949
DRIVER AMPLIFIER CHOICE
Although the AD7949 is easy to drive, the driver amplifier must
meet the following requirements:
•
•
•
Table 8. Recommended Driver Amplifiers
Amplifier
ADA4841-x
AD8655
AD8021
AD8022
OP184
AD8605,
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
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.
AD8615
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
IN
and C
IN
or by an external filter,
Rev. C | Page 18 of 32
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
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 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.
When using either the 2.5 V or the 4.096 V internal reference,
the internal temperature sensor is disabled. 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.
Figure 30. 2.5 V or 4.096 V Internal Reference Connection
10µF
AD7949
REF
TEMP
GND
REFIN
100nF
Data Sheet