REV. A
AD7677
–14–
For instance, a driver with an equivalent input noise of 2 nV/√Hz
(like the AD8021) and configured as a buffer, thus with a noise
gain of +1, the SNR degrades by only 0.07 dB with the filter in
Figure 5, and 0.27 dB without.
•
The driver needs to have a THD performance suitable to
that of the AD7677.
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 a dual version is needed
and gain of 1 is used.
The AD8132 or the AD8138 could also be used to generate a
differential signal from a single-ended signal.
The AD829 is another alternative where high frequency (above
1 MHz) performance is not required. In gain of 1, it requires an
82 pF compensation capacitor.
The AD8610 is also another option where low bias current is
needed in low frequency applications.
Voltage Reference Input
The AD7677 uses an external 2.5 V voltage reference. The
voltage reference input REF of the AD7677 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 1 µF ceramic
capacitor and a low ESR tantalum capacitor connected to the
REF and REFGND inputs with minimum parasitic inductance.
47 µF 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 AD7677s, 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, 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.
Note that V
REF
, as mentioned in the specification table, could be
increased to AVDD – 1.85 V. Since the input range is defined
in terms of V
REF
, this would essentially increase the range to
make it a ±3 V input range with a reference voltage of 3 V. One
of the benefits here is the increased SNR obtained as a result of
this increase. 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 improve-
ment is approximately 1 dB. The AD780 can be selected with a
3 V reference voltage.
FREQUENCY – Hz
75
PSRR – dB
35
65
10k 10M1k 1M
55
100k
45
70
60
50
40
Figure 9. PSRR vs. Frequency
Power Supply
The AD7677 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
AD7677 is independent of power supply sequencing once
OVDD does not exceed DVDD by more than 0.3 V, and thus
is free from supply voltage induced latchup. Additionally, it is
very insensitive to power supply variations over a wide fre-
quency range as shown in Figure 9.
POWER DISSIPATION
In Impulse Mode, the AD7677 automatically reduces its power
consumption at the end of each conversion phase. During the
acquisition phase, the operating currents are very low, which
allows a significant power saving when the conversion rate is
reduced as shown in Figure 10. This feature makes the AD7677
ideal for very low power battery applications.
It should be noted that the digital interface remains active even
during the acquisition phase. To reduce the operating digital
supply currents even further, the digital inputs need to be driven
close to the power rails (i.e., DVDD and DGND) and OVDD
should not exceed DVDD by more than 0.3 V.
SAMPLING RATE – SPS
1M
POWER DISSIPATION – W
0.1
10k
100 100k10 10k
100
1k
1
100k
1k
10
1M
WARP/NORMAL
IMPULSE
Figure 10. Power Dissipation vs. Sample Rate
