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Create analogue interfaces for low power design

Posted: 01 Oct 2013 ?? ?Print Version ?Bookmark and Share

Keywords:MCUs? low power? Analogue reference circuit? Low pass? filtering circuits?

A typical current requirement for a Vref input is 0.2 uA when the block is enabled but no conversion is taking place. During the conversion the reference input current increases to 0.1 mA or more. If multiple input channels are being converted it is likely that a simple 0.1 uF bypass capacitor will not be able to supply that current without the voltage on the input drooping.

To ensure the accuracy is not dependent on the bypass capacitor the value of the series impedance should be selected to allow the diode to maintain regulation even with conversion current flowing.

For example, if selected reference diode voltage is 2.5V with a 20 uA minimum regulation current, Iref is 0.1 mA and Avcc is 3.3V; then the series impedance should be no larger than (3.3V-2.5V)/120 uA or 6.7k Ohms. This also results in a 120 uA current draw even when the MCU is in a standby condition. Considering MCU standby currents are often less than 1 microamp this current is often not acceptable.

Figure 5: P-channel MOSFET (PMOS) controlled by a GPIO.

Figure 6: Using a MOSFET circuit to switch the reference circuit.

A MOSFET circuit can be used to switch the reference circuit when not in use the same way it was used for the sensor bias circuit. This connection is shown in figure 6.

Switching the reference circuit does require verifying the allowable Vref connnections for the MCU. Often the low side of the circuit is switched so Vref "idles" at Avcc, in this case it is important to ensure the MOSFET has a very low on resistance so it does not affect the reference accuracy. Also some Vref inputs have relatively high switching spikes so checking the dynamic drop across the MOSFET is important.

Since most reference diodes are lower power the current available to charge the capacitance in the circuit is typically low. This can result in fairly long delays, especially if the ADC block has requirements for a conversion start delay after applying Vref. Switching the reference circuit, whether on the high side or low side, allows reducing the value of Rref. This results in a lower average power dissipation on the diode since it is not continuously conducting. The lower resistance also helps reduce the ripple voltage and stabilisation time of the Vref input when switched.

Another option when a reference voltage is required is to connect the reference diode to an analogue input instead of Vref as shown in figure 7.

Figure 7: When a reference voltage is required, connect the reference diode to an analogue input.

The actual value of Vrefh can be calculated from the converted value of the AN0 input above since the voltage on the AN0 input is known. Since the analogue input is not a power input to the ADC block the value of Rref can be increased.

In the previous example, the reference circuit had a continuous current of 120 uA to supply both the diode current and Vref input, in this circuit that value can be reduced to 20 uA or the minimum regulation current of the diode. This is extremely useful especially if it is not desirable to switch the reference voltage circuit on and off. This circuit could also be switched from a GPIO or a sensor rail and since it is very low current it makes that design easier.

When using this configuration the actual voltage value of any sensed input can easily be calculated by dividing the ADC counts of that channel by the ADC counts of the reference channel then multiplying by the rated voltage of the reference diode

Many internal voltage references are connected this way, one advantage to this connection over supplying the reference diode input into the Vref pin is this allows converting voltages that are greater than the reference diode voltage but less than Avcc. For example, a very common reference diode voltage is 2.5V but the system voltage is 3.3V. Using this connection allows monitoring voltages up to 3.3V while still getting the accuracy of a reference diode.

On many MCUs, like the RX, the R2R ladder can be internally connected to Vcc. Often the Vref input then becomes another analogue input so this connection does not even consume another analogue input channel.

Sensor filtering
Sensors and other analogue input parameters are often connected to the MCU by cables or long trace distances due to the requirements of the parameter being sensed. If a thermistor is being used to measure ambient temperature then it is often not practical to have it on the PC board close to the MCU.

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