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| 型號： | AD7621AST |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | ADC |
| 英文描述： | 16-Bit, 1 LSB INL, 3 MSPS PulSAR ADC |
| 中文描述： | 1-CH 16-BIT SUCCESSIVE APPROXIMATION ADC, SERIAL/PARALLEL ACCESS, PQFP48 |
| 封裝： | MO-026-BBC, LQFP-48 |
| 文件頁數(shù)： | 18/26頁 |
| 文件大小： | 265K |
| 代理商： | AD7621AST |
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REV. Pr D
PRELIMINARY TECHNICAL DATA
AD7621
–18–
The noise generated by the driver amplifier needs to be
kept as low as possible in order to preserve the SNR
and transition noise performance of the AD7621. The
noise coming from the driver is filtered by the AD7621
analog input circuit one-pole low-pass filter made by
R
+
,
R
-
and C
S
. The SNR degradation due to the ampli-
fier is :
SNR
LOG
f
Ne
(
LOSS
N
=
+
)
20
56
3136
2
-3dB
,
where :
f
-3dB
is the -3dB input bandwidth in MHz of the AD7621
(50 MHz) or the cutoff frequency of the input filter if
any used
N is the noise factor of the amplifiers ( 1 if in buffer con-
figuration )
e
N
is the equivalent input noise voltage of each op-amp in
nV/(Hz)
1/2
For instance, a driver with an equivalent input noise of
2nV/
Hz
like the AD8021 and configured as a buffer, thus
with a noise gain of +1, the SNR degrades by only 0.17
dB with the filter in figure 5, and 0.8 dB without.
The driver needs to have a THD performance suitable
to that of the AD7621.
The
meets these requirements and is usuallycapacitor should have good linearity as an NPO ceramic
or mica type.
The
AD8022
could also be used where dual version is
needed and gain of 1 is used.
Single to Differential Driver
For applications using unipolar analog signals, a single-
ended to differential driver will allow for a differential
input into the part. The schematic is shown in Figure 8.
This configuration, when provided an input signal of 0 to
V
REF
, will produce a differential
V
REF
/2.
V
REF
with midscale at
If the application can tolerate more noise, the AD8138,
differential driver, can be used.
10pF
U2
590
AD8021
ANALOG INPUT
(UNIPOLAR 0 to 2.048V)
10pF
U1
590
AD8021
IN+
AD7621
IN-
1k
1k
REF
10 F
15
15
100nF
1.2nF
1.2nF
Figure 8. Single Ended to Differential Driver Circuit (
internal reference buffer used )
Voltage Reference
The AD7621 allows the choice of either a very low tem-
perature drift internal voltage reference or an external
reference.
Unlike many ADC with internal reference, the internal
reference of the AD7621 provides excellent performances
and can be used in almost all applications. It is tempera-
ture compensated to 1.2V ± TBD mV with a typical drift
of TBD ppm/°C, a typical long-term stability of TBD
ppm and a typical hysterisis of TBD ppm.
However, the advantages to use the external reference
voltage directly are :
- The power saving of about 8mW typical when the inter-
nal reference and its buffer are powered down ( PDREF
and PDBUF High )
- The SNR and dynamic range improvement of about 1.7
dB resulting of the use of a reference voltage very close to
the supply (2.5V) instead of a typical 2.048V reference
when the internal buffer is used.
To use the internal reference along with the internal
buffer, PDREF and PDBUF should both be LOW. This
will produce a voltage on REFBUFIN of 1.2 V and the
buffer will gain it up, resulting in a 2.048 V reference on
REF pin.
It is useful to decouple the REFBUFIN pin with a 100 nF
ceramic capacitor. The output impedance of the
REFBUFIN pin is 16 k . Thus, the 100 nF capacitor
provides an RC filter for noise reduction.
To use an external reference along with the internal
buffer, PDREF should be HIGH and PDBUF should be
low. This powers down the internal reference and allows
for the 1.2 V reference to be applied to REFBUFIN.
To use an external reference directly on REF pin,
PDREF and PDBUF should both be HIGH.
It should be noted that the internal reference and internal
buffer are independent of the power down (PD) pin of the
part. Furthermore, powering up the internal reference and
internal buffer requires time due to the charge of the REF
decoupling.
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