BPS Amperes PCB

🌍 Overview

This is the design document for the Amperes Board. As one of the four important boards within our system, we need to be able to track the current within the car so that it is safe and manageable. We also want to consider how we can record the current and apply the result to a State of Charge (SoC) algorithm.

Old Design

Open

Limited by the original chip's use of isoSPI, this design (credited to Tianda Huang) used a current sense amplifier to measure the current on the chip's low side.

 Design options

There are only two ways to measure current. We can measure the voltage across a resistor in series with the load, or we can measure the magnetic field of the wire. Invasive vs non-invasive

	Option 1 - Voltage Across a Resistor	Option 2 - Magnetic Field of Wire

	Option 1 - Voltage Across a Resistor	Option 2 - Magnetic Field of Wire
Overview	Current Sense Amplifier or Isolated Modulators/ADC	Hall-Effect Current Sensing
Screenshot	Open
Pros and cons	Accuracy Simplicity and cost Affected by electric noise

Decision 1

1. Option 1 - Making the PCB through a shunt resistor

Members can look into the second option for a hall-effect version in the future.

Ideas

Figure 1 is a standard method to measure current using a shunt-resistor. There are three steps to read the current:

1. The differential voltage is fed into the Current Sense Amplifier and converted to a single-ended signal.

2. This single-ended signal is connected to an ADC, digitizing the signal.

3. The signal is sent to a microcontroller for processing.

⚡ High Side vs Low Side

For reading the current, there are two different configurations you can have your device hooked up to.

See Introduction to Current Sense Amplifiers to learn more

Figure 2 shows an example of the current sense amplifier connected to the shunt resistor in a high-side sensing configuration.

Advantages:

• Able to detect load short to ground

• Current is monitored directly from the source

Disadvantages:

• High voltage can limit the variety of devices

Figure 3 shows an example of the current sense amplifier connected to the shunt resistor in a low-side sensing configuration.

Advantages:

• Wide range of available options

• It does not need an advanced sensor

Disadvantages:

• Difficult to detect load short to ground

Decision 2

1. Both?!: use high and low-side sensing

We will get the advantages of both and no disadvantages except cost. Plus, built-in fault monitoring.

🖼️ Chips to pick

You can choose different chips to make your life easier. The current sense amplifier is great, but you could also use an isolated modulator (an integrated ADC) to track the voltage differential. Below is the selected shunt resistor that was given as a requirement.

Current Shunt Resistor: WSBM8518L5000JK

• Resistance: 500 µOhms

📢 Amplifier types

There are many ways to measure the voltage across a resistor, as different components can perform different functions during assembly and testing. Below is a list of variations on calculating the same thing.

The Operational Amplifier (Op Amp) is the most straightforward building block of all the technologies listed below, as shown in Figure 4. An op amp amplifies the voltage difference between two input pins and outputs that voltage difference. Based on the resistor and wire combination connections you add to the inputs and outputs, you can mimic several behaviors. Unfortunately, it is challenging to configure these devices to handle a differential voltage accurately, as any selected resistors are super specific (5.121235 Ohms) and super expensive.

Current Sense Amplifiers are the next option for current sensing, like the example shown in Figure 5. These op amps have built-in specific resistor values that have been fine-tuned to handle these differential voltages at an affordable price.

Configurations include:

• High Side - Specialized to handle high-voltage differentials

• Low Side - Specialized to handle low-voltage differentials

• Bidirectional current - Specialized to measure in both directions

Isolated amplifiers have properties similar to current sense amplifiers, except for the benefit of physically separating the high and low-voltage sides, preventing issues like ground loops, shown in Figure 6. The component is more resistant to input and ground noise, which is helpful in noisy environments. They are also not limited to the high-low side configurations of current sense amplifiers, as they measure the voltage at any level, eliminating common-mode voltage limitations. It comes at the cost of accuracy due to the isolation barrier, similar to a hall effect sensor. This component is necessary as we work with a 120V battery while the PCBs use 3.3V-5V.

Isolated ADCs are the “All-in-one PCs, “similar to the isolated amplifier, as shown in Figure 7. Using this chip is almost too easy.

Advantages:

• Often includes integrated isolation barrier

• Digital output already compatible with MCUs

Disadvantages:

• Many are Sigma-Delta, which have inherent latency

• It can be 5-10x the cost of other solutions ($5-$10 for a single chip)

Decision 3

1. Battery → Shunt → Isolated Amplifier → ADC → MCU

We would like to minimize the latency of the components, and I don’t like all-in-one computers.

📃 Choice of Isolated Amplifier

I have decided to use the AMC1306M25. Here are the specifications and reasons why.

Input Specifications:

• Input voltage range: ±250mV (matches the 500μΩ shunt)

• Programmable Gain: 8 (default)

• Input common-mode range: -0.1V to +0.1V

Performance:

• Sample rate: 21 MSPS

• SNR: 82dB

• SFDR: 83dB

• Resolution: 16-bit

Isolation:

• Reinforced isolation: 5000 Vrms

• Working voltage: 1500 Vrms

• CMTI: 50 kV/μs (common mode transient immunity)

• Creepage/Clearance: 8.5mm

Interface/Power:

• Serial CMOS interface

• Supply voltage: 3.3V

• Power consumption: 42.4mW typical

• Temperature range: -40°C to 125°C

Physical:

• Package: SOIC-8

• Single channel

Cost: $1.80 (standard version)

These specs are particularly relevant because:

1. The input range matches the shunt resistor

2. Isolation specs exceed the 120V battery requirements

3. Sample rate and resolution are good for current monitoring

4. Power and interface compatible with standard MCUs

 Follow up

Vocab

Current Sense Amplifier: Amplifies the small voltage drop across a shunt resistor to measure current accurately in circuits.

Isolated Modulator: Converts analog signals to digital while maintaining electrical isolation, often used in high-voltage or noisy environments.

Hall-Effect Sensor: Detects magnetic fields to measure position, speed, or current without direct electrical contact.

Zero-drift: the phenomenon where a sensor's output signal shifts away from its baseline (or zero) value when there is no actual current flowing through the sensor; affects Hall-Effect Sensor

 Resource files

Open How to Sense Current	Open Hall-Effect White Paper	Open Shunt vs Hall-Effect	Open Isolated Amplifiers vs Isolated Modulators
Open Introduction to Current Sense Amplifiers