Datasheet

26 pF

4 pF

Conversion Phase - Switch Open

Track Phase - Switch Closed

ADC081S021

SNAS308G –APRIL 2005–REVISED MAY 2016

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Product Folder Links: ADC081S021

9 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers should

validate and test their design implementation to confirm system functionality.

9.1 Application Information

A typical application of the ADC is shown in Typical Application. Power is provided in this example by the Texas

Instruments LP2950 low-dropout voltage regulator (see LP295x-N Series of Adjustable Micropower Voltage

Regulators, SNVS764), available in a variety of fixed and adjustable output voltages. The power supply pin is

bypassed with a capacitor network placed close to the ADC. Because the reference for the ADC is the supply

voltage, any noise on the supply degrades 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.

9.1.1 Analog Inputs

An equivalent circuit for the ADC's input is shown in Figure 23. Diodes D1 and D2 provide ESD protection for the

analog inputs. At no time must the analog input go beyond (V

+ 300 mV) or (GND − 300 mV), as these ESD

diodes begin to conduct, which could result in erratic operation. For this reason, the ESD diodes must not be

used to clamp the input signal.

The capacitor C1 in Figure 23 has a typical value of 4 pF, and is mainly the package pin capacitance. Resistor

R1 is the ON resistance of the track or hold switch, and is typically 500 Ω. Capacitor C2 is the ADC sampling

capacitor and is typically 26 pF. The ADC delivers 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 23. Equivalent Input Circuit

9.1.2 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 5.25 V with respect to GND, regardless of V

, the supply voltage. This

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