AD7910ARM-REEL AD [Analog Devices], AD7910ARM-REEL Datasheet - Page 11

• Manufacturer Part Number: AD7910ARM-REEL
• Description: 250 kSPS, 10-/12-Bit ADCs in 6-Lead SC70
• Manufacturer: AD [Analog Devices]

When the ADC starts a conversion (see Figure 5), SW2 opens
and SW1 moves to position B, causing the comparator to become
unbalanced. The control logic and charge redistribution DAC are
used to add and subtract fixed amounts of charge from the
sampling capacitor to bring the comparator back into a balanced
condition. When the comparator is rebalanced, the conversion
is complete. The control logic generates the ADC output code.
Figure 6 shows the ADC transfer function.
ADC Transfer Function
The output coding of the AD7910/AD7920 is straight binary.
The designed code transitions occur at the successive integer
LSB values, i.e., 1 LSB, 2 LSBs, and so on. The LSB size is
V
ideal transfer characteristic for the AD7910/AD7920 is shown
in Figure 6.
Typical Connection Diagram
Figure 7 shows a typical connection diagram for the AD7910/
AD7920. V
should be well decoupled. This provides an analog input range of
0 V to V
four leading zeros followed by the MSB of the 12-bit or 10-bit
result. The 10-bit result from the AD7910 will be followed by two
trailing zeros.
REV. B
DD
V
IN
/4096 for the AD7920 and V
DD
SW1
A
. The conversion result is output in a 16-bit word with
AGND
REF
B
111...111
111...110
111...000
011...111
000...010
000...001
000...000
CAPACITOR
SAMPLING
Figure 5. ADC Conversion Phase
Figure 6. Transfer Characteristic
is taken internally from V
CONVERSION
PHASE
V
0V
DD
/2
1LSB
ANALOG INPUT
SW2
DD
1LSB = V
1LSB = V
/1024 for the AD7910. The
COMPARATOR
+V
DD
DD
DD
DD
/1024 (AD7910)
/4096 (AD7920)
and, as such, V
–1LSB
REDISTRIBUTION
CHARGE
CONTROL
DAC
LOGIC
DD
–11–
Alternatively, because the supply current required by the AD7910/
AD7920 is so low, a precision reference can be used as the
supply source to the AD7910/AD7920. An REF19x voltage
reference (REF195 for 5 V or REF193 for 3 V) can be used to
supply the required voltage to the ADC (see Figure 7). This con-
figuration is especially useful if the power supply is quite noisy
or if the system supply voltages are at a value other than 5 V or
3 V (e.g., 15 V). The REF19x will output a steady voltage to the
AD7910/AD7920. If the low dropout REF193 is used, the current
it needs to supply to the AD7910/AD7920 is typically 1.2 mA.
When the ADC is converting at a rate of 250 kSPS the REF193
needs to supply a maximum of 1.4 mA to the AD7910/AD7920.
The load regulation of the REF193 is typically 10 ppm/mA
(REF193, V
for the 1.4 mA drawn from it. This corresponds to a 0.057 LSB
error for the AD7920 with V
0.014 LSB error for the AD7910. For applications where power
consumption is of concern, the power-down mode of the ADC
and the sleep mode of the REF19x reference should be used to
improve power performance. See the Modes of Operation section.
Table I provides typical performance data with various references
used as a V
ature under the same setup conditions.
0V TO V
680nF
INPUT
DD
1.2mA
DD
Table I. AD7920 Typical Performance
for Various Voltage References IC
Reference
Tied to V
AD780 @ 3 V
REF193
AD780 @ 2.5 V
REF192
REF43
S
GND
V
Figure 7. REF193 as Power Supply
= 5 V), which results in an error of 14 ppm (42 mV)
V
IN
source for a 100 kHz input tone at room temper-
DD
0.1 F
AD7910/
AD7920/
DD
TANT
1 F
3V
DD
SDATA
SCLK
CS
= 3 V from the REF193 and a
REF193
AD7920 SNR
Performance (dB)
72.65
72.35
72.5
72.2
72.6
AD7910/AD7920
10 F
INTERFACE
SERIAL
0.1 F
5V
SUPPLY
C/ P