CS5560

18

DS713A5

7/31/07

The internal oscillator can be used if the signals to be measured are essentially DC. The internal oscillator

exhibits jitter at about 500 picoseconds rms. If the CS5560 is used to digitize AC signals, an external

low-jitter clock source should be used.

If the internal oscillator is used as the clock for the CS5560, the maximum conversion rate will be dictated

by the oscillator frequency.

3.4 Voltage Reference

The voltage reference for the CS5560 can range from 2.4 volts to 4.2 volts. A 4.096 volt reference is re-

quired to achieve the specified performance. Figure 7 and Figure 8 illustrate the connection of the voltage

reference with either a single +5 V analog supply or with ±2.5 V.

For optimum performance, the voltage reference device should be one that provides a capacitor connec-

tion to provide a means of noise filtering, or the output should include some type of bandwidth-limiting fil-

ter. Some 4.096 volt reference devices need only 5 volts total supply for operation and can be connected

as shown in Figure 7 or Figure 8. The reference should have a local bypass capacitor and an appropriate

output capacitor.

Some older 4.096 voltage reference designs require more headroom and must operate from an input volt-

age of 5.5 to 6.5 volts. If this type of voltage reference is used ensure that when power is applied to the

system, the voltage reference rise time is slower than the rise time of the V1+ and V1- power supply volt-

age to the converter. An example circuit to slow the output startup time of the reference is illustrated in

Figure 6.

Figure 6. Voltage Reference Circuit

3.5 Analog Input

The analog input of the converter is fully differential with a peak input of 4.096 volts on each input. This is

illustrated in Figure 7 and Figure 8. These diagrams also illustrate a differential buffer amplifier configura-

tion for driving the CS5560.

The capacitors at the outputs of the amplifiers provide a charge reservoir for the dynamic current from the

A/D inputs while the resistors isolate the dynamic current from the amplifier. The amplifiers can be pow-

ered from higher supplies than those used by the A/D but precautions should be taken to ensure that the

op amp output voltage remains within the power supply limits of the A/D, especially under start-up condi-

tions.

2k

10

µF

5.5 to 15 V

VIN

VOUT

GND

4.096 V

Refer to V1- and VREF1 pins.