Mixed-Signal Voltage Reference Level Boosting

Here is an example of the charge spikes that can appear on the voltage reference pin of a modern ADC during conversion. The upper trace (Trace 4) is the converter's start conversion signal . As shown in Figure 1, different amounts of charge are required at the ADC's voltage reference pin (Trace 1) during the conversion process. In this figure, the oscilloscope's low capacitance probe captures the voltage drop that appears across a 10 kOhm resistor between the ADC's voltage reference pin input and the voltage reference output. The ADC's voltage reference must be able to accommodate these high frequency charge spikes.

Figure 1. Current flowing into and out of a modern ADC reference input pin.

Voltage references are available in either a two-terminal shunt or three-terminal series configuration. Figure 2 shows the difference between these two voltage reference options. The name of the reference shown in Figure 2 (a) (two-terminal shunt voltage reference) means that the entire shunt reference IC chip operates in parallel with its load. With a shunt voltage reference, you apply an input voltage to a resistor connected to the cathode. Typical initial voltage accuracy for this device can be as low as 0.5% to 5%, and the temperature coefficient is about 50 to 100 mV/°C. Shunt voltage references may be suitable for converters up to 8 bits.

Figure 2: Two-terminal shunt (a) and three-terminal series (b) voltage reference structures

The series voltage reference shown in Figure 2 (b) operates in series with its load. An internal bandgap voltage, along with an internal amplifier, creates the output voltage of this reference. The series voltage reference generates an output voltage between the output and ground while providing the appropriate output current to the external load. As the load current increases or decreases, the series reference maintains the voltage at VOUT.

The typical initial voltage accuracy of a series reference can be as low as 0.05% to 0.5%, with a temperature coefficient as high as 2.5 ppm/°C. Due to the excellent initial voltage and over-temperature performance of the series reference, we can use this device when driving the reference pins of precision ADCs and DACs. Beyond 8 (LSB size = 0.39%) or 14 (LSB size = 0.006%) bits of resolution, an external series voltage reference ensures that you can use the converter to achieve the desired accuracy.