AD7946 Analog Devices, AD7946 Datasheet - Page 14

• Manufacturer Part Number: AD7946
• Manufacturer: Analog Devices

Specifications of AD7946

• Resolution (bits): 14bit
• # Chan: 1
• Sample Rate: 500kSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: (Vref) p-p
• Adc Architecture: SAR
• Pkg Type: CSP,SOP

AD7946
tolerated. The THD degrades as a function of the source
impedance and the maximum input frequency, as shown in
Figure 28.
DRIVER AMPLIFIER CHOICE
Although the AD7946 is easy to drive, the driver amplifier
needs to meet the following requirements:
•
•
•
Figure 28. THD vs. Analog Input Frequency and Source Resistance
The noise generated by the driver amplifier needs to be
kept as low as possible to preserve the SNR and transition
noise performance of the AD7946. Note that the AD7946
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
coming from the amplifier is filtered by the AD7946 analog
input circuit 1-pole, low-pass filter made by R
by the external filter, if one is used.
For ac applications, the driver should have a THD perform-
ance commensurate with the AD7946. Figure 17 shows the
THD vs. frequency that the driver should exceed.
For multichannel multiplexed applications, the driver
amplifier and the AD7946 analog input circuit must settle a
full-scale step onto the capacitor array at a 14-bit level
(0.006%). In the amplifier’s data sheet, settling at 0.1% to
0.01% is more commonly specified. This could differ
significantly from the settling time at a 14-bit level and
should be verified prior to driver selection.
–100
–105
–110
–80
–85
–90
–95
0
R
R
R
R
S
S
S
S
= 250Ω
=100Ω
= 50Ω
= 33Ω
25
FREQUENCY (kHz)
50
75
IN
and C
100
IN
Rev. A | Page 14 of 24
or
Table 8. Recommended Driver Amplifiers
Amplifier
ADA4841
AD8021
AD8655
AD8022
OP184
AD8605,
AD8519
AD8031
VOLTAGE REFERENCE INPUT
The AD7946 voltage reference 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, as explained in the Layout section.
When REF is driven by a very low impedance source, for
example, a reference buffer using the
10 μF (X5R, 0805 size) ceramic chip capacitor is appropriate for
optimum performance.
If an unbuffered reference voltage is used, the decoupling value
depends on the reference used. For instance, a 22 μF (X5R,
1206 size) ceramic chip capacitor is appropriate for optimum
performance using a low temperature drift ADR43x reference.
If desired, smaller reference decoupling capacitor values down
to 2.2 μF can be used with a minimal impact on performance,
especially DNL.
Regardless, there is no need for an additional lower value ceramic
decoupling capacitor (for example, 100 nF) between the REF
and GND pins.
AD8615
Typical Application
Very low noise, small and low power
Very low noise and high frequency
5 V single-supply, low noise, and low power
Low noise and high frequency
Low power, low noise, and low frequency
5 V single-supply, low power
Small, low power, and low frequency
High frequency and low power
AD8031
or the AD8603, a