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| 型號(hào)： | AD7665ASTRL |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | ADC |
| 英文描述： | 16-Bit, 570 kSPS CMOS ADC |
| 中文描述： | 4-CH 16-BIT SUCCESSIVE APPROXIMATION ADC, SERIAL/PARALLEL ACCESS, PQFP48 |
| 封裝： | LOW PROFILE, MS-026BBC, QFP-48 |
| 文件頁(yè)數(shù)： | 15/24頁(yè) |
| 文件大小： | 348K |
| 代理商： | AD7665ASTRL |
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REV. 0
AD7665
–15–
FREQUENCY
–
kHz
–
110
0
100
T
–
100
–
90
–
80
–
70
1000
R = 50
R = 11
R = 100
Figure 8. THD vs. Analog Input Frequency and Input
Resistance (0 V to 2.5 V Only)
Driver Amplifier Choice
Although the AD7665 is easy to drive, the driver amplifier needs
to meet at least the following requirements:
The driver amplifier and the AD7665 analog input circuit
must be able, together, to settle for a full-scale step the capaci-
tor array at a 16-bit level (0.0015%). In the amplifier’s data
sheet, the settling at 0.1% to 0.01% is more commonly speci-
fied. It could significantly differ from the settling time at
16-bit level and it should therefore be verified prior to the
driver selection. The tiny op amp AD8021, which combines
ultralow noise and a high-gain bandwidth, meets this settling
time requirement even when used with a high gain up to 13.
The noise generated by the driver amplifier needs to be kept
as low as possible in order to preserve the SNR and transi-
tion noise performance of the AD7665. The noise coming
from the driver is first scaled down by the resistive scaler
according to the analog input voltage range used, and is then
filtered by the AD7665 analog input circuit one-pole, low-
pass filter made by (R/2 + R1) and C
S
. The SNR degradation
due to the amplifier is:
SNR
f
N e
FSR
LOSS
dB
N
=
+
20
28
784
2
2 5
1000
3
2
log
.
–
π
where
f
–3
dB
is the –3 dB input bandwidth of the AD7665 (3.6 MHz)
or the cut-off frequency of the input filter if any used
(0 V to 2.5 V range).
is the noise factor of the amplifier (1 if in buffer
configuration).
is the equivalent input noise voltage of the op amp
in nV/(Hz)
1/2
.
is the full-scale span (i.e., 5 V for
±
2.5 V range).
For instance, when using the 0 V to 2.5 V range, a driver like
the AD8021, with an equivalent input noise of 2 nV/
√
Hz
and
configured as a buffer, thus with a noise gain of 1, the SNR
degrades by only 0.12 dB.
N
e
N
FSR
The driver needs to have a THD performance suitable to
that of the AD7665. TPC 8 gives the THD versus frequency
that the driver should preferably exceed.
The AD8021 meets these requirements and is usually appropri-
ate for almost all applications. The AD8021 needs an external
compensation capacitor of 10 pF. This capacitor 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.
The AD829 is another alternative where high-frequency (above
100 kHz) performance is not required. In gain of 1, it requires
an 82 pF compensation capacitor.
The AD8610 is another option where low bias current is needed
in low frequency applications.
Voltage Reference Input
The AD7665 uses an external 2.5 V voltage reference. The
voltage reference input REF of the AD7665 has a dynamic
input impedance. Therefore, it should be driven by a low
impedance source with an efficient decoupling between REF
and REFGND inputs. This decoupling depends on the choice
of the voltage reference, but usually consists of a low ESR tanta-
lum capacitor connected to the REF and REFGND inputs with
minimum parasitic inductance. 47
μ
F is an appropriate value for
the tantalum capacitor when used with one of the recommended
reference voltages:
– The low-noise, low-temperature drift ADR421 and AD780
voltage references.
– The low-power ADR291 voltage reference.
– The low-cost AD1582 voltage reference.
For applications using multiple AD7665s, it is more effective to
buffer the reference voltage with a low-noise, very stable op amp
such as the AD8031.
Care should also be taken with the reference temperature coeffi-
cient of the voltage reference which directly affects the full-scale
accuracy if this parameter matters. For instance, a
±
15 ppm/
°
C
tempco of the reference changes the full scale by
±
1 LSB/
°
C.
Scaler Reference Input (Bipolar Input Ranges)
When using the AD7665 with bipolar input ranges, the connec-
tion diagram in Figure 5 shows a reference buffer amplifier.
This buffer amplifier is required to isolate the REFIN pin from
the signal dependent current in the AIN pin. A high-speed op
amp such as the AD8031 can be used with a single 5 V power
supply without degrading the performance of the AD7665. The
buffer must have good settling characteristics and provide low
total noise within the input bandwidth of the AD7665.
Power Supply
The AD7665 uses three sets of power supply pins: an analog
5 V supply AVDD, a digital 5 V core supply DVDD, and a
digital input/output interface supply OVDD. The OVDD supply
allows direct interface with any logic working between 2.7 V and
5.25 V. To reduce the number of supplies needed, the digital
core (DVDD) can be supplied through a simple RC filter from
the analog supply as shown in Figure 5. The AD7665 is inde-
pendent of power supply sequencing and thus free from supply
voltage induced latchup. Additionally, it is very insensitive to
power supply variations over a wide frequency range as shown
in Figure 9.
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