AD7661
Rev. 0 | Page 19 of 28
• 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 AD7661. The noise
coming from the driver is filtered by the AD7661 analog
input circuit 1-pole low-pass filter made by R1 and C2 or
by the external filter, if one is used. The SNR degradation
due to the amplifier is
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π
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log20
N
dB
LOSS
Nef
SNR
where:
f
–3dB
is the input bandwidth, in MHz, of the AD7661 (0.82)
or the cutoff frequency of the input filter, if one is
used.
N is the noise factor of the amplifier (+1 in buffer
configuration).
e
N
is the equivalent input noise voltage of the op amp, in
nV/√Hz.
For example, the OP184 driver, which has an equivalent
input noise of 4 nV/√Hz and a noise gain of +1 when
configured as a buffer, degrades the SNR by only 0.11 dB.
•
The driver needs to have a THD performance suitable to
that of the AD7661. Figure 15 gives the THD versus
frequency that the driver should exceed.
The OP184, OP162 or AD8519 meet these requirements and are
usually appropriate for almost all applications. As an alternative,
in very high speed and noise-sensitive applications, the AD8021
with an external 10 pF compensation capacitor can be used.
This capacitor 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 AD8022 could also be used if a dual version is needed
and gain of +1 is present. The AD829 is an alternative in
applications where high frequency (above 100 kHz)
performance is not required. In gain of +1 applications, it
requires an 82 pF compensation capacitor. The AD8610 is
an option when low bias current is needed in low
frequency applications.
Voltage Reference Input
The AD7661 allows the choice of either a very low temperature
drift internal voltage reference or an external 2.5 V reference.
Unlike many ADCs with internal references, the internal
reference of the AD7661 provides excellent performance and
can be used in almost all applications.
To use the internal reference along with the internal buffer,
PDREF and PDBUF should both be LOW. This will produce
1.2 V on REFBUFIN which, amplified by the buffer, will result
in a 2.5 V reference on the REF pin.
The output impedance of REFBUFIN is 11 k
Ω (minimum) when
the internal reference is enabled. It is necessary to
decouple
REFBUFIN with a ceramic capacitor greater than 10 nF. Thus
the 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 the 2.5 V
reference to be applied to REFBUFIN.
To use an external reference directly on REF pin, PDREF and
PDBUF should both be HIGH.
PDREF and PDBUF power down the internal reference and the
internal reference buffer, respectively. Note that the PDREF and
PDBUF input current should never exceed 20 mA. This could
eventually occur when input voltage is above AVDD (for
instance at power up). In this case, a 100 Ω series resistor is
recommended.
The internal reference is temperature compensated to 2.5 V ±
20 mV. The reference is trimmed to provide a typical drift of
3 ppm/°C . This typical drift characteristic is shown in Figure
22. For improved drift performance, an external reference, such
as the
AD780, can be used.
The AD7661 voltage reference input REF has a dynamic input
impedance; it should therefore be driven by a low impedance
source with efficient decoupling between the REF and
REFGND inputs. This decoupling depends on the choice of the
voltage reference but usually consists of a low ESR tantalum
capacitor connected to REF and REFGND with minimum
parasitic inductance. A 10 µF (X5R, 1206 size) ceramic chip
capacitor (or 47 µF tantalum capacitor) is appropriate when
using either the internal reference or one of these
recommended reference voltages:
•
The low noise, low temperature drift ADR421 and AD780
•
The low power ADR291
•
The low cost AD1582
