ADCS7477 datasheet(21/33 Pages) TI1

21 / 33 page

ADCS7476, ADCS7477, ADCS7478

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SNAS192G – APRIL 2003 – REVISED MAY 2016

Product Folder Links: ADCS7476 ADCS7477 ADCS7478

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8 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.

8.1 Application Information

A typical application of ADCS747x is shown in Figure 32. The combined analog and digital supplies are provided

in this example by the TI LP2950 low-dropout voltage regulator, available in a variety of fixed and adjustable

output voltages. The supply is bypassed with a capacitor network located close to the device. The three-wire

interface is also shown connected to a microprocessor or DSP.

8.1.1 Analog Inputs

An equivalent circuit for the ADCS747x input channel is shown in Figure 31. The diodes D1 and D2 provide ESD

protection for the analog inputs. At no time should an analog input exceed VDD + 300 mV or GND – 300 mV, as

these ESD diodes begin conducting current into the substrate or supply line and affect ADC operation.

The capacitor C1 in Figure 31 typically has a value of 4 pF, and is mainly due to pin capacitance. The resistor R1

represents the ON resistance of the multiplexer and track or hold switch, and is typically 100

Ω. The capacitor C2

is the ADCS747x sampling capacitor, and is typically 26 pF.

The sampling nature of the analog input causes input current pulses that result in voltage spikes at the input.

ADCS747x delivers best performance when driven by a low-impedance source to eliminate distortion caused by

the charging of the sampling capacitance. In some applications where dynamic performance is critical, the input

must be driven with a low output-impedance amplifier. In addition, when using ADCS747x to sample AC signals,

a band-pass or low-pass filter reduces harmonics and noise and thus improve THD and SNR.

Figure 31. Equivalent Input Circuit

8.1.2 Digital Inputs and Outputs

The ADCS747x digital inputs (SCLK and CS) are not limited by the same absolute maximum ratings as the

analog inputs. The digital input pins are instead limited to 6.5 V with respect to GND, regardless of VDD, the

supply voltage. This allows ADCS747x to be interfaced with a wide range of logic levels, independent of the

supply voltage.

NOTE

Even though the digital inputs are tolerant of up to 6.5 V above GND, the digital outputs

are only capable of driving VDD out.

In addition, the digital input pins are not prone to latch-up; SCLK and CS may be asserted before VDD without

any risk.

ADCS7477 Datasheet(PDF) 21 Page - Texas Instruments