AnalogTalk

You are an experienced design engineer and have done your job well for over 15 years.  Due to new requirements coming down from management in order to obtain government funding, you are suddenly being required to include power measurement in your projects.  How do you measure power, and do so accurately?  Obviously it requires a couple A/D converters to measure voltage and current and a simple current sensor such as a shunt could be used.  However the signals are so small that a very accurate A/D converter must be used.  Is a 16-bit ADC good enough?

To answer this question, let’s start with the most basic assertion: Resolution does not equate to accuracy.  You can have a 16-bit resolution ADC but only have 13-bit accurate measurements.  There are many specifications that can be viewed, but when it comes to actually designing your application, what you need to know is the smallest signal level you need to measure to obtain your accurate measurements.

Let’s start with the current measurement as that is where the biggest dynamic range of the application will occur.  To use a power measurement example, you know from the application requirements that the maximum current input will be 50A.  You also learn that you need at least 2% accuracy at 5A input as the minimum requirement

To determine the smallest signal that we need to measure, let’s look at the smallest current input error.  We know that at 5A we can only have 2% error.  In this case it is equal to 100mA.  This would be the smallest detectable current to be measured in the application.  For practical purposes you would select a value less than this to give some extra head room.

The maximum current input to your application will determine the size of the current shunt.  If the current values are too large, often a shunt cannot be used due to heating.  For 50A maximum current input, a small shunt of 400 µohms can be used.  That would give a voltage reading of 20 mV.  Remember that the smallest current we need to detect is 100mA.  Using our 400 µohm shunt, we would read a value of 40 µV.  That is a very small value.

So how do I determine if my ADC will be able to meet these requirements?  Stay tuned for part 2 when we discuss ADC specifications and how they relate to your measurements.

In the previous post we talked about the charging and discharging behavior or a Li-ion battery.   On our way to a full fledged fuel gauge we need to first measure battery voltage.   Figure 1 shows the battery voltage measurement circuit using and ADC, in this case a MCP3421 18-bit ADC (U1).

Figure 1 Battery Voltage Measurement

Since the MCP3421 device has an internal reference voltage, the measurable maximum input voltage range is limited to the internal voltage reference voltage of up to 2.048V.  To measure the input voltage higher than the internal reference, a voltage divider is used, which is formed by R1, R2, and R3.  The R3 is optional and is used to calibrate the R1 and R2 component tolerance.  By choosing the series resistance value of the voltage divider to be very high (> 1 MΩ), the current losses due to the voltage divider is negligible.

In the example circuit as shown in Figure 1, the ADC is configured as single ended by connecting the positive input pin (VIN+) to the battery voltage, while the negative input pin (VIN-) to the VSS.   The ADC output is available to the MCU via the I2C bus line.
Figure 2 shows the discharge curve of a 3.7V Li-Polymer battery (3.7V, 170 mAH).  The curve shows that the battery voltage reduces linearly until it reaches about 80% of its full capacity.

Figure 2 Li-Polymer Battery Voltage Discharging Curve
Since the battery discharging characteristics are very linear until the point where the curve falls off sharply, measuring only the battery voltage is an alternative low-cost method to estimate the current status of the battery. In this case, the measured battery voltage can be compared with the fuel values in the lookup table in the MCU firmware.
The circuit shown can be used for measuring the battery voltage of any battery type. When the circuit is used, the voltage divider (R1, R2, R3) must be properly adjusted in order to keep the maximum input voltage (or the voltage at VIN+ pin when the battery is fully charged) to the ADC device is less than the ADC internal reference voltage (2.048V).
Although using the voltage alone is not sufficient to represent the battery fuel status, this method is widely used for simple and cost-sensitive applications because of its straightforward implementation.

Related Links

Battery Management

Delta-Sigma Analog-to-Digital Converters