Datasheet

IN1
IN2
MICROPROCESSOR
DSP
SCLK
CS
DIN
DOUT
GND
V
A
ADC122S051
LP2950
5V
1 PF
TANT
0.1 PF 0.1 PF1 PF
| |
|
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
ADC122S051
www.ti.com SNAS257E –NOVEMBER 2004–REVISED MARCH 2013
Figure 49. Ideal Transfer Characteristic
TYPICAL APPLICATION CIRCUIT
A typical application of the ADC122S051 is shown in Figure 50. Power is provided, in this example, by the TI
LP2950 low-dropout voltage regulator, available in a variety of fixed and adjustable output voltages. The power
supply pin is bypassed with a capacitor network located close to the ADC122S051. Because the reference for
the ADC122S051 is the supply voltage, any noise on the supply will degrade device noise performance. To keep
noise off the supply, use a dedicated linear regulator for this device, or provide sufficient decoupling from other
circuitry to keep noise off the ADC122S051 supply pin. Because of the ADC122S051's low power requirements,
it is also possible to use a precision reference as a power supply to maximize performance. The four-wire
interface is shown connected to a microprocessor or DSP.
Figure 50. Typical Application Circuit
ANALOG INPUTS
An equivalent circuit for one of the ADC122S051's input channels is shown in Figure 51. Diodes D1 and D2
provide ESD protection for the analog inputs. At no time should any input go beyond (V
A
+ 300 mV) or (GND −
300 mV), as these ESD diodes will begin conducting, which could result in erratic operation. For this reason,
these ESD diodes should NOT be used to clamp the input signal.
The capacitor C1 in Figure 51 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
ADC122S051 sampling capacitor and is typically 30 pF. The ADC122S051 will deliver best performance when
driven by a low-impedance source to eliminate distortion caused by the charging of the sampling capacitance.
This is especially important when using the ADC122S051 to sample AC signals. Also important when sampling
dynamic signals is a band-pass or low-pass filter to reduce harmonics and noise, improving dynamic
performance.
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