D/A Converter Outputs - Z-World Wildcat BL2000 User Manual

C-programmable, with ethernet

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3.6 D/A Converter Outputs

Figure 18 shows the analog voltage reference circuit.

100 W

DAC_PWR

4.096_VREF

+ V

JP3

3 2

100 nF

453 W

14 kW

1.707_VREF

10 kW

100 nF

Figure 18. Analog Reference Voltages

This circuit generates the 4.096 V reference voltage, which is used by the A/D converter

and optionally by the two D/A converters. This sets the operating range of the A/D con-

verter and the D/A converters (0–4.096 V). To use the full accuracy of the A/D converter

and the D/A converters, this voltage must be accurate to the same degree.

Under normal operation, the 453 Ω resistor is not installed. The reference zener diode in

combination with the 100 Ω resistor form a shunt regulator. The 4.096 V reference voltage

then feeds the A/D converter, the D/A converters, and the voltage divider composed of the

10 kΩ and the 14 kΩ resistors. The voltage divider generates a second reference voltage of

1.707 V to feed the four op-amps for the buffered A/D converter inputs.

The reference voltage can be ratiometric rather than absolute. This is done by removing

the zener diode and installing the 453 Ω resistor. With this arrangement, the reference

voltages follow changes in the power supply voltages Vcc and V+, which is a filtered ver-

sion of Vcc. This type of measurement circuit is preferred by some customers whose sen-

sors are powered from the Vcc supply and hence the outputs track Vcc.

A jumper on header JP3 allows the D/A converters to be powered either from the 4.096 V

reference (factory default) or from the analog supply +V. The D/A converters use their

power source also as the reference input, so normally powering the D/A converters from

the more accurate 4.096 V reference is best. However, should a customer desire more

dynamic range (0–5 V rather than 0–4.096 V), the jumper across JP3 can be set to power

the D/A converters from +V. When powered from the +V supply, the outputs of the D/A

converters will always be ratiometric, independent of whether the zener diode is installed.

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