Datasheet

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0V
+V
A
- 1.5 LSB
0.5 LSB
ANALOG INPUT
1 LSB = V
A
/4096
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
ADC121C021, ADC121C021Q, ADC121C027
SNAS415F –JANUARY 2008–REVISED MARCH 2013 www.ti.com
ANALOG INPUT
An equivalent circuit for the input of the ADC121C021 is shown in Figure 22. The diodes provide ESD protection
for the analog input. The operating range for the analog input is 0 V to V
A
. Going beyond this range will cause
the ESD diodes to conduct and may result in erratic operation. For this reason, these diodes should NOT be
used to clamp the input signal.
The capacitor C1 in Figure 22 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
ADC121C021 sampling capacitor, and is typically 30 pF. The ADC121C021 will deliver best performance when
driven by a low-impedance source (less than 100Ω). This is especially important when using the ADC121C021 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 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 ADC121C021 is straight binary. Code transitions occur midway between successive
integer LSB values. The LSB width for the ADC121C021 is V
A
/ 4096. 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
/ 8192. Other code transitions occur at intervals of 1 LSB.
Figure 23. Ideal Transfer Characteristic
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Product Folder Links: ADC121C021 ADC121C021Q ADC121C027