Datasheet

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

provide ESD protection for the analog inputs. At no time should an analog input exceed V

+ 300 mV or GND -

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

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

, the

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

of the supply voltage.

Product Folder Links: ADCS7476 ADCS7477 ADCS7478