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| 型号： | EVAL-AD7665EDZ |
| 厂商： | Analog Devices Inc |
| 文件页数： | 7/23页 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL FOR AD76XXEDZ |
| 标准包装： | 1 |
| 系列： | * |
REV.
AD7665
–15–
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 capacitor
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 specified.
It could significantly differ from the settling time at a 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 CS. The SNR degradation due
to the amplifier is
SNR
f
Ne
FSR
LOSS
dB
N
=
+
20
28
784
2
25
3
2
log
.
–
p
where:
f–3 dB is the –3 dB input bandwidth in MHz of the AD7665
(3.6 MHz) or the cutoff frequency of the input filter
if any used (0 V to 2.5 V range).
N
is the noise factor of the amplifier (1 if in buffer
configuration).
eN
is the equivalent input noise voltage of the op amp
in nV/Hz
1/2.
FSR 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.
∑ The driver needs to have a THD performance suitable to
that of the AD7665. TPC 11 gives the THD versus frequency
that the driver should preferably exceed.
The AD8021 meets these requirements and is usually appropriate
for almost all applications. The AD8021 needs an external com-
pensation capacitor of 10 pF. This capacitor should have good
linearity as an NPO ceramic or mica type.
The AD8022 could also be used where a dual version is needed
and a gain of 1 is used.
The AD829 is another alternative where high frequency (above
100 kHz) performance is not required. In a 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; it should therefore 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 1 F ceramic capacitor and a
low ESR tantalum capacitor connected to the REF and REFGND
inputs with minimum parasitic inductance. 47 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
like the AD8031.
Care should also be taken with the reference temperature coeffi-
cient of the voltage reference that 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.
Note that VREF , as mentioned in the Specification tables, could
be increased to AVDD – 1.85 V. The benefit here is the increased
SNR obtained as a result of this increase. Since the input range
is defined in terms of VREF, this would essentially increase the
±REF range from ±2.5 V to ±3 V and so on with an AVDD above
4.85 V. The theoretical improvement as a result of this increase
in reference is 1.58 dB (20 log [3/2.5]). Due to the theoretical
quantization noise, however, the observed improvement is approxi-
mately 1 dB. The AD780 can be selected with a 3 V reference
voltage.
Scaler Reference Input (Bipolar Input Ranges)
When using the AD7665 with bipolar input ranges, the connection
diagram in Figure 5 shows a reference buffer amplifier. This
buffer amplifier is required to isolate the REF pin from the signal
dependent current in the INx 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
5V 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 DVDD
+ 0.3 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 indepen-
dent of power supply sequencing, once OVDD does not exceed
DVDD by more than 0.3 V, and thus free from supply voltage
induced latch-up. Additionally, it is very insensitive to power
supply variations over a wide frequency range as shown in Figure 9.
C
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