Datasheet

V
IN
D1
R1
C2
30 pF
V
A
D2
C1
3 pF
Conversion Phase - Switch Open
Track Phase - Switch Closed
| |
|
0V
+V
A
- 1.5LSB
0.5LSB
ANALOG INPUT
1LSB = V
A
/4096
ADC CODE
111...111
111...110
111...000
011...111
000...010
000...001
000...000
ADC128S022
www.ti.com SNAS334E –AUGUST 2005–REVISED MARCH 2013
Figure 37. Ideal Transfer Characteristic
ANALOG INPUTS
An equivalent circuit for one of the ADC128S022's input channels is shown in Figure 38. Diodes D1 and D2
provide ESD protection for the analog inputs. The operating range for the analog inputs is 0 V to V
A
. Going
beyond this range will cause the ESD diodes to conduct and result in erratic operation.
The capacitor C1 in Figure 38 has a typical value of 3 pF and is mainly the package pin capacitance. Resistor R1
is the on resistance of the multiplexer and track / hold switch and is typically 500 ohms. Capacitor C2 is the
ADC128S022 sampling capacitor, and is typically 30 pF. The ADC128S022 will deliver best performance when
driven by a low-impedance source (less than 100 ohms). This is especially important when using the
ADC128S022 to sample dynamic signals. Also important when sampling dynamic signals is a band-pass or low-
pass filter which reduces harmonics and noise in the input. These filters are often referred to as anti-aliasing
filters.
Figure 38. Equivalent Input Circuit
DIGITAL INPUTS AND OUTPUTS
The ADC128S022's digital inputs (SCLK, CS, and DIN) have an operating range of -0.3 V to V
A
. They are not
prone to latch-up and may be asserted before the digital supply (V
D
) without any risk. The digital output (DOUT)
operating range is controlled by V
D
. The output high voltage is V
D
- 0.5V (min) while the output low voltage is
0.4V (max).
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