Datasheet

| |
|
0V
+V
A
- 1.5 LSB
0.5 LSB
ANALOG INPUT
1 LSB = V
A
/256
ADC CODE
111...111
111...110
111...000
011...111
000...010
000...001
000...000
V
IN
D1
R1
C2
30 pF
V
A
D2
C1
3 pF
Conversion Phase - Switch Open
Track Phase - Switch Closed
ADC081C021, ADC081C027
SNAS447C –FEBRUARY 2008–REVISED MARCH 2013 www.ti.com
Figure 22. Equivalent Input Circuit
The analog input is sampled for eight internal clock cycles, or for typically 400 ns, after the fall of SDA for
acknowledgement. This time could be as long as about 530 ns. The sampling switch opens and the conversion
begins this time after the fall of ACK. This time are typical at room temperature and may vary with temperature.
ADC TRANSFER FUNCTION
The output format of the ADC081C021 is straight binary. Code transitions occur midway between successive
integer LSB values. The LSB width for the ADC081C021 is V
A
/ 256. The ideal transfer characteristic is shown in
Figure 23. The transition from an output code of 0000 0000 0000 to a code of 0000 0000 0001 is at 1/2 LSB, or a
voltage of V
A
/ 512. Other code transitions occur at intervals of 1 LSB.
Figure 23. Ideal Transfer Characteristic
REFERENCE VOLTAGE
The ADC081C021 uses the supply (V
A
) as the reference, so V
A
must be treated as a reference. The analog-to-
digital conversion will only be as precise as the reference (V
A
), so the supply voltage should be free of noise. The
reference should be driven by a low output impedance voltage source.
The Applications section provides recommended ways to provide the supply voltage appropriately. Refer to
TYPICAL APPLICATION CIRCUIT for details.
16 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated
Product Folder Links: ADC081C021 ADC081C027