AD7949 Analog Devices, AD7949 Datasheet - Page 19

• 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

Data Sheet
External Reference and Internal Buffer
For improved drift performance, an external reference can be
used with the internal buffer, as shown in Figure 31. The
external source is connected to REFIN, the input to the on-chip
unity gain buffer, and the output is produced on the REF pin.
For applications requiring the use of the temperature sensor, an
external reference must be used, with or without the internal
buffer. The internal temperature sensor measures the internal
temperature of the
system calibration. Refer to Table 9 for register details. With the
buffer enabled, the gain is unity and is limited to an input/output
of VDD = −0.2 V; however, the maximum voltage allowable
must be ≤(VDD − 0.5 V).
The internal reference buffer is useful in multiconverter appli-
cations 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 architecture of the AD7949.
External Reference
In any of the six voltage reference schemes, an external reference
can be connected directly on the REF pin as shown in Figure 32
because the output impedance of REF is >5 kΩ. To reduce power
consumption, the reference and buffer should be powered down.
When using only the external reference (and optional reference
buffer as shown in Figure 35), the internal buffer is disabled.
Refer to Table 9 for register details. For improved drift perfor-
mance, an external reference such as the
recommended.
Note that the best SNR is achieved with a 5 V external reference
as the internal reference is limited to 4.096 V. The SNR
degradation is as follows:
SNR
LOSS
Figure 31. External Reference Using Internal Buffer
=
20
AD7949
10µF
log
AD7949
Figure 32. External Reference
. 4
10µF
AD7949
096
5
REF
TEMP
GND
and is thus useful for performing a
REF SOURCE
0.5V < REF < (VDD + 0.3V)
≤ (VDD – 0.5V)
REF SOURCE
REF
REFIN
NO CONNECTION
REQUIRED
TEMP
GND
REFIN
100nF
ADR43x
or
ADR44x
Rev. C | Page 19 of 32
is
Reference Decoupling
Whether using an internal or external reference, the AD7949
voltage reference output/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. This decoupling depends on the choice of the voltage
reference but usually consists of a low ESR capacitor connected
to REF and GND with minimum parasitic inductance. A 10 μF
(X5R, 1206 size) ceramic chip capacitor is appropriate when
using the internal reference, the ADR43x/ADR44x external
reference, or a low impedance buffer such as the
the AD8605.
The placement of the reference decoupling capacitor is also impor-
tant to the performance of the AD7949, as explained in the
Layout section. Mount the decoupling capacitor on the same side
as the ADC at the REF pin with a thick PCB trace. The GND
should also be connected to the reference decoupling capacitor
with the shortest distance and to the analog ground plane with
several vias.
If desired, smaller reference decoupling capacitor values down
to 2.2 μF can be used with minimal impact on performance,
especially on DNL.
Regardless, there is no need for an additional lower value ceramic
decoupling capacitor (for example, 100 nF) between the REF
and GND pins.
For applications that use multiple AD7949 devices or other
PulSAR devices, it is more effective to use the internal reference
buffer to buffer the external reference voltage, thus reducing
SAR conversion crosstalk.
The voltage reference temperature coefficient (TC) directly
impacts full scale; therefore, in applications where full-scale
accuracy matters, care must be taken with the TC. For instance,
a ±10 ppm/°C TC of the reference changes full scale by ±1 LSB/°C.
AD8031
AD7949
or