AD7699 Analog Devices, AD7699 Datasheet - Page 19

• Manufacturer Part Number: AD7699
• Description: 16-Bit, 8-Channel, 500 kSPS PulSAR ADC
• Manufacturer: Analog Devices

Specifications of AD7699

• Resolution (bits): 16bit
• # Chan: 8
• Sample Rate: 500kSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni,SE-Bip,SE-Uni
• Ain Range: (Vref) p-p,4 V p-p,Bip (Vref) x 0.5,Uni (Vref)
• Adc Architecture: SAR
• Pkg Type: CSP

Data Sheet
Reference Decoupling
Whether using an internal or external reference, the
voltage reference output/input, REF, has a dynamic input
impedance and should therefore be driven by a low impedance
source with efficient decoupling between the REF and GND
pins. This decoupling depends on the choice of the voltage
reference but usually consists of a low ESR capacitor connected
to REF and GND with minimum parasitic inductance. A 10 μF
(X5R, 1206 size) ceramic chip capacitor is appropriate when using
the internal reference, the ADR43x/ADR44x external reference,
or a low impedance buffer such as the
The placement of the reference decoupling capacitor is also
important to the performance of the AD7699, as explained in the
Layout section. Mount the decoupling capacitor on the same side as
the ADC at the REF pin with a thick PCB trace. The GND should
also connect to the reference decoupling capacitor with the shortest
distance and to the analog ground plane with several vias.
If desired, smaller reference decoupling capacitor values down
to 2.2 μF can be used with a minimal impact on performance,
especially on DNL.
Regardless, there is no need for an additional lower value ceramic
decoupling capacitor (for example, 100 nF) between the REF
and GND pins.
For applications that use multiple AD7699s or other PulSAR
devices, it is more effective to use the internal reference buffer
to buffer the external reference voltage, thus reducing SAR
conversion crosstalk.
The voltage reference temperature coefficient (TC) directly
impacts full scale; therefore, in applications where full-scale
accuracy matters, care must be taken with the TC. For instance,
a ±15 ppm/°C TC of the reference changes full scale by ±1 LSB/°C.
POWER SUPPLY
The
core supply (VDD) and a digital input/output interface supply
(VIO). VIO allows direct interface with any logic between 1.8 V
and VDD. To reduce the supplies needed, the VIO and VDD pins
can be tied together. The
sequencing between VIO and VDD. The only restriction is that
CNV must be low when powering up the AD7699. Additionally,
it is very insensitive to power supply variations over a wide
frequency range, as shown in Figure 32.
AD7699
uses two power supply pins: an analog and digital
AD7699
is independent of power supply
AD8031
or the AD8605.
AD7699
Rev. A | Page 19 of 28
The
conversion phase; therefore, the operating currents and power
scale linearly with the sampling rate. This makes the part ideal
for low sampling rates (even of a few hertz) and low battery-
powered applications.
SUPPLYING THE ADC FROM THE REFERENCE
For simplified applications, the AD7699, with its low operating
current, can be supplied directly using the reference circuit, as
shown in Figure 34. The reference line can be driven by
•
•
•
1
5V
OPTIONAL REFERENCE BUFFER AND FILTER.
AD7699
The system power supply directly
A reference voltage with enough current output capability,
such as the
A reference buffer, such as the AD8605, which can also
filter the system power supply, as shown in Figure 34
10,000
10kΩ
0.010
0.001
1000
1µF
75
70
65
60
55
50
45
40
35
30
100
0.1
10
1
1
10
Figure 33. Operating Currents vs. Sampling Rate
VDD = 5V, INTERNAL REF
powers down automatically at the end of each
Figure 34. Example of an Application Circuit
5V
ADR43x/ADR44x
AD8605
VDD = 5V, EXTERNAL REF
100
Figure 32. PSRR vs. Frequency
10
1
SAMPLING RATE (sps)
FREQUENCY (kHz)
10µF
1k
5V
100
REF
10k
10Ω
1µF
AD7699
VIO
VDD
1k
0.1µF
100k
AD7699
VIO
0.1µF
10k
1M