ADCS7476, ADCS7477, ADCS7478

SNAS192F – APRIL 2003 – REVISED MARCH 2013

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TYPICAL APPLICATION CIRCUIT

A typical application of the ADCS7476/77/78 is shown in Figure 30. 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.

Figure 30. Typical Application Circuit

ANALOG INPUTS

An equivalent circuit for the ADCS7476/77/78 input channel is shown in Figure 31. The diodes D1 and D2

300 mV, as these ESD diodes will 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 / hold switch, and is typically 100 ohms. The capacitor

C2 is the ADCS7476/77/78 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. The

ADCS7476/77/78 will deliver best performance when driven by a low-impedance source to eliminate distortion

caused by the charging of the sampling capacitance. In applications where dynamic performance is critical, the

input might need to be driven with a low output-impedance amplifier. In addition, when using the

ADCS7476/77/78 to sample AC signals, a band-pass or low-pass filter will reduce harmonics and noise and thus

improve THD and SNR.

Figure 31. Equivalent Input Circuit

DIGITAL INPUTS AND OUTPUTS

The ADCS7476/77/78 digital inputs (SCLK and CS) are not limited by the same absolute maximum ratings as the

supply voltage. This allows the ADCS7476/77/78 to be interfaced with a wide range of logic levels, independent

of the supply voltage.

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