Datasheet

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0V
+V
A
- 1.5 LSB
0.5 LSB
ANALOG INPUT
1 LSB = V
A
/1024
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
ADC101C021, ADC101C027
SNAS446D –FEBRUARY 2008–REVISED FEBRUARY 2013 www.ti.com
The capacitor C1 in Figure 19 has a typical value of 3 pF and is mainly the package pin capacitance. Resistor R1
is the on resistance (R
ON
) of the multiplexer and track / hold switch and is typically 500Ω. Capacitor C2 is the
ADC101C021 sampling capacitor and is typically 30 pF. The ADC101C021 will deliver best performance when
driven by a low-impedance source (less than 100Ω). This is especially important when using the ADC101C021 to
sample dynamic signals. A buffer amplifier may be necessary to limit source impedance. Use a precision op-amp
to maximize circuit performance. Also important when sampling dynamic signals is a band-pass or low-pass filter
to reduce noise at the input.
Figure 19. 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 ADC101C021 is straight binary. Code transitions occur midway between successive
integer LSB values. The LSB width for the ADC101C021 is V
A
/ 1024. The ideal transfer characteristic is shown
in Figure 20. 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
/ 2048. Other code transitions occur at intervals of 1 LSB.
Figure 20. Ideal Transfer Characteristic
REFERENCE VOLTAGE
The ADC101C021 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
Section TYPICAL APPLICATION CIRCUIT for details.
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Product Folder Links: ADC101C021 ADC101C027