Now that we have figured out the smallest signal we need to measure, let’s look at what the 16-bit ADCs of the MCP3901 can do for the energy measurement requirements. We need to look at more than resolution in order to determine if this is good enough for the application. If resolution were the only parameter to look at, we could simply calculate that over the input range of 1V, we would spread the 16-bits of resolution (65,536 codes) would give us a least significant bit, or LSB of 15.2µV. That sounds good, but does it give us our actual smallest detectable voltage?
Let’s look at some other parameters that could help us. One of these is distortion, which is typically characterized by the parameter THD or total harmonic distortion as introduced by the ADC that was not present in the original signal. Harmonic distortion results from spurious signals created from multiples of the fundamental frequency. It is specified as a ratio in dB of the highest harmonic spurious signal relative to the carrier frequency. The MCP3901 shows a specification of -104dBc THD. It would be smaller with a smaller input voltage. Similar devices from other suppliers typically have a THD of -90dBc or smaller. Naturally a larger value for THD is better, indicating a smaller value for the spurious harmonic signal.
Harmonic distortion is not the only parameter to look at to determining the accuracy of the ADCs. In order to include noise introduced by the ADCs, THD+Noise is used. A more common view of this parameter is the inverse of THD+Noise, called SINAD or signal in noise and distortion. The MCP3901 shows a SINAD of 91 dB. With a full scale input of 1V, the smallest detectable signal would be 28µV. Similar 16-bit resolution devices from other suppliers typically have a SINAD specification of 60-70 dB, indicating much higher noise. On the higher end of 70 dB, this would yield a smallest detectable signal of 316µV. This is a significant difference in accuracy between devices with the same resolution.
Looking at specifications other than resolution, it appears than specifications such as THD and SINAD can be very revealing with respect to the actual accuracy of the ADCs being used to make measurements. Looking at the specifications of the MCP3901 the 28µV detectable signal threshold with a 1V input signal is more than adequate for the 40µV application requirement. This is much better than what might be obtained from similar 16-bit resolution ADCs from other suppliers, which could not even meet the basic application requirements.
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.
This video gives a brief overview of the MCP3901 ADC Evaluation Board for 16-bit MCUs. The board is a complete tool for the evaluation of the MCP3901 analog front end. The video briefly describes the functionality and applications of the MCP3901 which is ideally suited for energy measurement or power monitoring applications. It also shows the main functionality of the board including graphical user interfaces, board connections and included MCU modules.
Figure 2 Li-Polymer Battery Voltage Discharging Curve