Datasheet

V
IN
D1
R1
C2
26 pF
V
A
D2
C1
4 pF
Conversion Phase - Switch Open
Track Phase - Switch Closed
MICROPROCESSOR
DSP
SCLK
CS
SDATA
GND
V
A
ADC081S021
LP2950
5V
1 PF
0.1 PF 0.1 PF1 PF
V
IN
ADC081S021
SNAS308E –APRIL 2005–REVISED MARCH 2013 www.ti.com
TYPICAL APPLICATION CIRCUIT
A typical application of the ADC is shown in Figure 21. Power is provided in this example by the Texas
Instruments 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 ADC. Because the reference for
the ADC 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 ADC supply pin. Because of the ADC's low power requirements, it is also possible to use a
precision reference as a power supply to maximize performance. The three-wire interface is shown connected to
a microprocessor or DSP.
Figure 21. Typical Application Circuit
ANALOG INPUTS
An equivalent circuit for the ADC's input is shown in Figure 22. Diodes D1 and D2 provide ESD protection for the
analog inputs. At no time should the analog input go beyond (V
A
+ 300 mV) or (GND − 300 mV), as these ESD
diodes will begin to conduct, which could result in erratic operation. For this reason, the ESD diodes should not
be used to clamp the input signal.
The capacitor C1 in Figure 22 has a typical value of 4 pF, and is mainly the package pin capacitance. Resistor
R1 is the on resistance of the track / hold switch, and is typically 500 ohms. Capacitor C2 is the ADC sampling
capacitor and is typically 26 pF. The ADC 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 ADC to sample AC signals. Also important when sampling dynamic signals is an anti-aliasing filter.
Figure 22. Equivalent Input Circuit
DIGITAL INPUTS AND OUTPUTS
The ADC digital inputs (SCLK and CS) are not limited by the same maximum ratings as the analog inputs. The
digital input pins are instead limited to +5.25V with respect to GND, regardless of V
A
, the supply voltage. This
allows the ADC to be interfaced with a wide range of logic levels, independent of the supply voltage.
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