AD7951 Analog Devices, AD7951 Datasheet - Page 21

• Manufacturer Part Number: AD7951
• Manufacturer: Analog Devices

Specifications of AD7951

• Resolution (bits): 14bit
• # Chan: 1
• Sample Rate: 1MSPS
• Interface: Byte,Par,Ser,SPI
• Analog Input Type: Diff-Bip,Diff-Uni
• Ain Range: Bip 10V,Bip 5.0V,Uni 10V,Uni 5.0V
• Adc Architecture: SAR
• Pkg Type: CSP,QFP

DRIVER AMPLIFIER CHOICE
Although the AD7951 is easy to drive, the driver amplifier must
meet the following requirements:
•
•
•
For multichannel, multiplexed applications, the driver
amplifier and the AD7951 analog input circuit must be
able to settle for a full-scale step of the capacitor array at a
14-bit level (0.006%). For the amplifier, settling at 0.1% to
0.01% is more commonly specified. This differs significantly
from the settling time at a 14-bit level and should be
verified prior to driver selection. The
bines ultralow noise and high gain bandwidth and meets
this settling time requirement even when used with gains
of up to 13.
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 AD7951. The noise coming from
the driver is filtered by the external one-pole low-pass filter
as shown in Figure 27. The SNR degradation due to the
amplifier is
where:
V
f
N is the noise factor of the amplifier (+1 in buffer
configuration).
e
The driver needs to have a THD performance suitable to
that of the AD7951. Figure 15 shows the THD vs. frequency
that the driver should exceed.
V
amp, in nV/√Hz.
SNR
–3dB
N
NADC
NADC
is the equivalent input voltage noise density of the op
is the cutoff frequency of the input filter (3.9 MHz).
LOSS
is the noise of the ADC, which is:
=
=
10
20
V
2
INp-p
SNR
log
20
2
⎛
⎜
⎜
⎜
⎜
⎜
⎝
V
NADC
2
+
V
NADC
π
2
f
−
AD8021
3
dB
(
Ne
N
op amp com-
)
2
⎞
⎟
⎟
⎟
⎟
⎟
⎠
Rev. 0 | Page 21 of 32
The AD8021 meets these requirements and is appropriate for
almost all applications. The AD8021 needs a 10 pF external
compensation capacitor that should have good linearity as an
NPO ceramic or mica type. Moreover, the use of a noninverting
+1 gain arrangement is recommended and helps to obtain the
best signal-to-noise ratio.
The
and a gain of 1 is present. The
tions where high frequency (above 100 kHz) performance is not
required. In applications with a gain of 1, an 82 pF compensation
capacitor is required. The
current is needed in low frequency applications.
Since the AD7951 uses a large geometry, high voltage input
switch, the best linearity performance is obtained when using
the amplifier at its maximum full power bandwidth. Gaining
the amplifier to make use of the more dynamic range of the
ADC results in increased linearity errors. For applications
requiring more resolution, the use of an additional amplifier
with gain should precede a unity follower driving the AD7951.
See Table 8 for a list of recommended op amps.
Table 8. Recommended Driver Amplifiers
Amplifier
ADA4841-x
AD829
AD8021
AD8022
AD8610/
AD8620
VOLTAGE REFERENCE INPUT/OUTPUT
The AD7951 allows the choice of either a very low temperature
drift internal voltage reference, an external reference, or an
external buffered reference.
The internal reference of the AD7951 provides excellent perform-
ance and can be used in almost all applications. However, the
linearity performance is guaranteed only with an external reference.
AD8022
can also be used when a dual version is needed
Typical Application
12 V supply, very low noise, low distortion,
low power, low frequency
±15 V supplies, very low noise, low frequency
±12 V supplies, very low noise, high frequency
±12 V supplies, very low noise, high
frequency, dual
±13 V supplies, low bias current, low
frequency, single/dual
AD8610
AD829
is an option when low bias
is an alternative in applica-
AD7951