D1

R1

C2

26 pF

D2

C1

4 pF

Conversion Phase - Switch Open

Track Phase - Switch Closed

MICROPROCESSOR

DSP

SCLK

CS

SDATA

GND

ADC101S021

LP2950

5V

1 PF

0.1 PF

0.1 PF

1 PF

ADC101S021

www.ti.com

SNAS307F – JULY 2005 – REVISED MARCH 2013

Typical Application Circuit

A typical application of the ADC is shown in Figure 19. 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 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 19. Typical Application Circuit

Analog Inputs

An equivalent circuit for the ADC's input is shown in Figure 20. Diodes D1 and D2 provide ESD protection for the

diodes will begin conducting, 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 20 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

Ω. 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 20. 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

allows the ADC to be interfaced with a wide range of logic levels, independent of the supply voltage.

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