Building analog interfaces for low power designEditor's note: Because modern high speed, low power MCUs are now so power efficient, analog circuits have become the major consumer of power in the system power budget. Here are some techniques for bringing the analog power budget into sync with the rest of the design.
Tremendous progress has been made in designing low power microcontrollers which consume only nanowatts. However, there is still a significant challenge in designing low power systems which interface with analog circuits and sensors. The challenge is made tougher by the increased accuracy and speed of the analog interfaces.
The low power landscape has changed dramatically in a very short period of time. The push for wireless and networked products has resulted in semiconductor manufacturers putting a lot of development resource into low power devices. In the microcontroller industry this has resulted in MCUs that achieve impressive low power numbers but also provide very high performance. Illustrative of that are a few of the key specifications for the Renesas RX111 microcontroller:
- 32 bit, 32 MHz with zero wait state flash
- 1.8V to 3.6V operation
- Operating currents (typical)
- 32 MHz, 3.3V – 10 mA with SPI and timers running
- Standby with Real Time Clock (RTC) – 690 nA
- Standby without RTC – 350 nA
- Wake-up time <5 uS
- Low voltage detect (LVD) only an additional 100 nA
These high speed, low power devices are so power efficient that analog circuit power
consumption that was once acceptable is now a major source of the system power budget. Figure 1 shows a typical MCU and sensor interface. In the simple circuit there are four distinct areas to consider which can affect the overall power budget.
- MCU - The MCU is always a primary factor in overall power consumption. Historically, this also was probably the largest portion of the power budget. The MCU may still be the largest power consumption circuit but with modern low-power MCUs the analog circuits can easily have a larger impact on the design.
- Analog reference circuit - The reference voltage circuit for the ADC can be a significant power drain. As seen in the figure the circuit is always energized and since it is a form of shunt regulator it must have a minimum current flowing through the reference device to maintain the regulated voltage.
- Sensor bias - The thermistor circuit is part of a voltage divider scheme. This circuit will also be a continuous current drain if implemented as shown.
- Low Pass or Filtering circuits - In many cases sensors will need noise filtering, especially for 60 Hz pickup, and may need gain. The active low pass filter circuit shown provides very good filtering and can provide some gain but the amplifier will require supply current. There are many very low power OpAmps available but they do add cost to the design.
Figure 1: Typical MCU and sensor interface
Sensor bias considerations
The diagram in Figure 1 showed the need for a reference diode circuit for the ADC. The reference circuit is not always required if the analog sensing is all ratiometric. Figure 2 shows a few different ADC connections and identifies whether they are ratiometric or not. A and B are ratiometric since the AD input voltage is a fractional portion of Vref.
The sensing input does not need to be a resistive element, it is just important that the voltage across it is a fraction of Vref and changes based on the sensed value. It also does not matter if Vref is connected to Vcc of the MCU or a separate Vref. Figures C and D are non-ratiometric since the AD input voltage does not necessarily change based upon a change in Vref.
Figure 2: Some ADC connections in which A and B are ratiometric and C and D are non-ratiometric
Ratiometric sensing typically reduces cost since the reference is not required, eliminating the reference circuit also will usually reduce the power required. In many MCUs there are two power inputs to the ADC block, AVcc and Vrefh. When ratiometric sensing is used both can be supplied from the MCU power rail.
Typically, AVcc supplies the ADC control and conversion circuits and Vref supplies the R2R conversion ladder circuit. The power these circuits require is the same whether the conversion is ratiometric or not, however, when the sensing is ratiometric there is no additional current required for the reference diode or IC which can range from tens to hundreds of microamps.
In a ratiometric sensing circuit the sensing divider will draw a bias current. This current can be significant for a low power application. If R1 and the thermistor in Figure 3 were both 15K and the system voltage was 3V there would be a continuous 100 uA drain just due to the sensor bias current.
Figure 3: In a ratiometric sensing circuit the sensing divider will draw a bias current.
A thermistor with a higher value could be selected and R1 could be increased but this can cause some issues with accurate sampling. This is due to the sample-and-hold (SH) circuit which is implemented in almost all current successive approximation converters in MCUs. The SH capacitor is connected to the input to be sampled for a very short period of time, especially in high speed ADC converters.
During the time the hold capacitor is connected it must charge to the voltage on the analog input, this charging will occur through R1. As R1 is increased to reduce sensor bias current the available current to charge the SH circuit decreases. So there is a constant trade-off between increasing the value of R1 and ADC accuracy. A small capacitor can be added on the analog input which will provide a low impedance source of charge for the SH circuit. This capacitor will help with the SH charge time issue but is a filter so it can introduce error if the ADC input is changing rapidly.
One good way to minimize the power consumed in the sensor circuit is to switch off the bias voltage to the divider when it is not in use. In the picture shown above the voltage to the sensing
divider is controlled by a General Purpose Input/Output (GPIO) pin. This is a very cost effective
solution if the MCU has enough GPIO but it can introduce some error since the voltage output of the GPIO is probably not exactly equal to Vrefh. Remember that ratiometric sensing relies on a
divider connected between Vrefh and Vrefl. Since the current is very low in the sensing circuit
and the MCU port outputs are MOSFETs the difference may be acceptable, especially if the
required precision is not too high.
Another concern with switching the bias to the sensor circuit on and off is there will be a delay to charge any capacitance in the circuit (Figure 4). In this case, C3 is providing the low impedance path to offset the SH circuit sampling window requirement but it must be charged before the conversion can begin. If C3 is 1 nanofarad and R1 is 15k Ohm the delay could be more than 80 microseconds.
Figure 5 shows using a P-channel MOSFET (PMOS) controlled by a GPIO to provide a better bias circuit switch. Using a discrete MOSFET allows the designer to select the device so the switch resistance (RDSon) does not impact the accuracy of the circuit. Another advantage of the MOSFET circuit is the voltage divider is now powered from the Vrefh power rail instead of the internal power rail of the MCU which can be noisy if other pins on the device are switching.
Figure 5: P-channel MOSFET (PMOS) controlled by a GPIO
When the GPIO is configured as an input port Q1 is biased off by R2. When the GPIO is set as an output and driven low Q1 will turn on, supplying bias voltage to the divider.
The MOSFET could be changed to an N-Channel device and connected on the low side of the divider. There is no significant advantage to one configuration over the other unless the sensed voltage is always closer to Vcc or Ground; if that is the case then having C3 biased to that charge state will reduce stabilization time since this configuration still relies on waiting for C3 to charge.
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