Datasheet

AD7655
Rev. B | Page 17 of 28
Table 8. Recommended Driver Amplifiers
Amplifier Typical Application
ADA4841
Very low noise, low distortion, low power,
low frequency
AD829 Very low noise, low frequency
AD8021 Very low noise, high frequency
AD8022 Very low noise, high frequency, dual
AD8605/AD8606/
AD8608/AD8615/
AD8616/ AD8618
5 V single supply, low power,
low frequency, single/dual/quad
AD8610/AD8620
Low bias current, low frequency,
single/dual
VOLTAGE REFERENCE INPUT
The AD7655 requires an external 2.5 V reference. The reference
input should be applied to REF, REFA, and REFB. The voltage
reference input REF of the AD7655 has a dynamic input
impedance; it should therefore be driven by a low impedance
source with an efficient decoupling. 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 REFA, REFB, and REFGND inputs with
minimum parasitic inductance. A value of 47 μF is appropriate
for the tantalum capacitor when using one of the recommended
reference voltages:
• The low noise, low temperature drift AD780, ADR421,
and ADR431 voltage references
• The low cost AD1582 voltage reference
For applications using multiple AD7655s with one voltage
reference source, it is recommended that the reference source
drives each ADC in a star configuration with individual
decoupling placed as close as possible to the REF/REFGND
inputs. Also, it is recommended that a buffer, such as the
AD8031/AD8032, be used in this configuration.
Care should be taken with the reference temperature coefficient
of the voltage reference, which directly affects the full-scale
accuracy if this parameter is applicable. For instance, a
15 ppm/°C tempco of the reference changes the full-scale
accuracy by 1 LSB/°C.
POWER SUPPLY
The AD7655 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 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 17.
The AD7655 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 latch-up. Additionally, it is
very insensitive to power supply variations over a wide
frequency range, as shown in
Figure 19.
FREQUENCY (kHz)
40
PSRR (dB)
100 1000 10000
45
50
55
60
65
70
10
1
03536-019
Figure 19. PSRR vs. Frequency
POWER DISSIPATION
In impulse mode, the AD7655 automatically reduces its power
consumption at the end of each conversion phase. During the
acquisition phase, the operating currents are very low, which
allows significant power savings when the conversion rate is
reduced, as shown in
Figure 20. This feature makes the AD7655
ideal for very low power battery applications.
Note 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 (that is, DVDD and DGND), and OVDD
should not exceed DVDD by more than 0.3 V.
SAMPLING RATE (kSPS)
0.1
POWER DISSIPATION (mW)
100 1000
1
10
100
1000
NORMAL
IMPULSE
03536-020
101
Figure 20. Power Dissipation vs. Sample Rate