Datasheet

AD7667
Rev. 0 | Page 20 of 28
For applications that use multiple AD7667s, it is more effective
to use the internal buffer to buffer the reference voltage.
Care should be taken with the voltage reference’s temperature
coefficient, which directly affects the full-scale accuracy if this
parameter matters. For instance, a ±15 ppm/°C temperature
coefficient of the reference changes full scale by ±1 LSB/°C.
Note that V
REF
can 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 0 V to 3 V with an AVDD above 4.85 V.
The AD780 can be selected with a 3 V reference voltage.
The TEMP pin, which measures the temperature of the
AD7667, can be used as shown in Figure 30. The output of
TEMP pin is applied to one of the inputs of the analog switch
(e.g., ADG779), and the ADC itself is used to measure its own
temperature. This configuration is very useful for improving the
calibration accuracy over the temperature range.
ADG779
AD8021
C
C
03035-0-024
ANALOG INPUT
(UNIPOLAR)
AD7667
IN
TEMPERATURE
SENSOR
TEMP
Figure 30. Temperature Sensor Connection Diagram
Power Supply
The AD7667 uses three power supply pins: an analog 5 V supply
AVDD, a digital 5 V core supply DVDD, and a digital input/
output interface supply OVDD. OVDD allows direct interface
with any logic between 2.7 V and DVDD + 0.3 V. To reduce the
supplies needed, the digital core (DVDD) can be supplied
through a simple RC filter from the analog supply, as shown in
Figure 26. The AD7667 is independent of power supply
sequencing once OVDD does not exceed DVDD by more than
0.3 V, and is thus free of supply voltage induced latch-up.
Additionally, it is very insensitive to power supply variations
over a wide frequency range, as shown in Figure 31, which
represents PSRR over frequency with on chip and external
references.
03035-0-031
INT REF
EXT REF
30
35
40
45
50
55
60
65
70
75
80
1 10 100 1000 10000
FREQUENCY (kHz)
PSRR (dB)
Figure 31. PSRR vs. Frequency
POWER DISSIPATION VERSUS THROUGHPUT
When using the Impulse mode of operation (IMPULSE =
HIGH, WARP = LOW), operating currents are very low during
the acquisition phase, allowing significant power savings when
the conversion rate is reduced (see Figure 32). The AD7667
automatically reduces its power consumption at the end of each
conversion phase. This makes the part ideal for very low power
battery applications. The digital interface and the reference
remain active even during the acquisition phase. To reduce
operating digital supply currents even further, digital inputs
need to be driven close to the power supply rails (i.e., DVDD or
DGND), and OVDD should not exceed DVDD by more than
0.3 V.
03035-0-032
10
100
1k
10k
1M
10 100 1k 10k 100k 1M
SAMPLING RATE (SPS)
POWER DISSIPATION (µW)
WARP MODE POWER
PDREF = PDBUF = HIGH
100k
IMPULSE MODE POWER
Figure 32. Power Dissipation vs. Sampling Rate