AD7467BRT AD [Analog Devices], AD7467BRT Datasheet - Page 10

• Manufacturer Part Number: AD7467BRT
• Description: 1.8 V, Micro-Power, 8/10/12-Bit ADCs in 6 Lead SOT-23
• Manufacturer: AD [Analog Devices]

0V toV DD
AD7466/AD7467/AD7468
The AD7468 conversion result consists of four leading
zeros followed by the MSB of the 8-bit result.
Alternatively, because the supply current required by the
AD7466/AD7467/AD7468 is so low, a presision reference
can be used as the supply source to the AD7466/AD7467/
AD7468. A REF19x voltage reference (REF193 for 3V,
REF192 for 2.5 V ) can be used to supply the required
voltage to the ADC - see figure 11. This configuration is
especially useful if your power supply is quite noisy or if
the system supply voltages are at some value other than 3V
(e.g. 2.5V).
the AD7466/AD7467/AD7468. In applications where
power consumption is important, the automatic power
down mode of the ADC and the sleep mode of the
REF19x reference should be used to improve power per-
formance. See Modes of Operation section of the
datasheet.
TYPICAL CONNECTION DIAGRAM
Figure 11 shows a typical connection diagram for the
AD7466/AD7467/AD7468. V
V
vides an analog input range of 0V to V
AD7466 the conversion result is output in a 16-bit word
with four leading zeroes followed by the MSB of the 12-
bit result. The AD7467 conversion result consists of four
leading zeros followed by the MSB of the 10-bit result.
680nF
INPUT
DD
Figure 11. REF193 as Power Supply to AD7466/AD7467/
and as such V
1mA
V IN
GND
V
DD
0.1µF
The REF19x will output a steady voltage to
AD7466/67/68
DD
1µF
TANT
+3V
should be well decoupled. This pro-
SDATA
SCLK
AD7468
REF193
REF
SERIAL
INTERFACE
is taken internally from
10µF
DD
0.1µF
. For the
pecifications
SUPPLY
µC/µP
+5V
–10–
Analog Input
Figure 12 shows an equivalent circuit of the analog input
sturcture of the AD7466/AD7467/7468. The two diodes
D1 and D2 provide ESD protection for the analog inputs.
Care must be taken to ensure that the analog input signal
never exceeds the supply rails by more than 200mV. This
will cause these diodes to become forward biased and start
conducting current into the substrate. The capacitor C1 in
figure 12 is typically about 4pF and can primarily be at-
tributed to pin capacitance. The resistor R1 is a lumped
component made up of the on resistance of a switch. This
resistor is typically about 100 . The capacitor C2 is the
ADC sampling capacitor and has a capacitance of 16pF
typically. For ac applications, removing high frequency
components from the analog input signal is recommended
by use of a band-pass filter on the relevant analog input
pin. In applications where harmonic distortion and signal
to noise ratio are critical the analog input should be driven
from a low impedance source. Large source impedances
will significantly affect the ac performance of the ADC.
This may necessitate the use of an input buffer amplifier.
The choice of the op amp will be a function of the par-
ticular application.
When no amplifier is used to drive the analog input the
source impedance should be limited to low values. The
maximum source impedance will depend on the amount of
total harmonic distortion (THD) that can be tolerated.
The THD will increase as the source impedance increases
and performance will degrade. Figure 13 shows a graph of
the Total Harmonic Distortion vs. analog input signal
frequency for different source impedances when using a
supply voltage of 2.7V and sampling at a rate of 100
kSPS.
V IN
C1
4pF
Figure 12. Equivalent Analog Input Circuit
V DD
D1
D2
CONVERSION PHASE - SWITCH OPEN
TRACK PHASE - SWITCH CLOSED
R1
16PF
C2
REV. PrC