DRV8353RS-EVM: Voltage Divider Impedance Too High for ADC Input of TMS320F28027?

Tool/software:

Hi,

We are currently evaluating a 3-phase BLDC motor using the following evaluation boards:

• Gate driver board: DRV8353RS-EVM

• MCU board: ISO-F28027F, featuring the TMS320F28027 microcontroller

In the DRV8353RS-EVM schematic, each motor phase voltage is scaled down using a resistor divider (383kΩ / 9.76kΩ) and fed into the ADC pins of the TMS320F28027.

According to the TMS320F2802x datasheet (Figure 9-20), the recommended source impedance for ADC inputs should be 50Ω or less to ensure proper sampling of the internal capacitor. The current resistor network results in an impedance of approximately 9.5kΩ, which seems significantly higher than recommended.

Additionally, we observe some voltage discrepancies—for example, when 24V is applied, the monitored voltage sometimes reads as only 22V. We suspect the high impedance might be the cause.

To improve ADC accuracy, we are considering reducing the divider to something like 5.1kΩ / 130Ω or even 2.2kΩ / 56Ω, which would bring the source impedance closer to the recommended value.

However, since this divider taps into the motor phase lines, we are concerned that lowering the resistor values too much could affect the signal integrity or load the phase outputs.

Could you please advise:

1. Is the currently implemented 383kΩ / 9.76kΩ divider appropriate for use with the TMS320F28027 ADC?

2. Would reducing the resistors to the 5.1kΩ / 130Ω or 2.2kΩ / 56Ω range be recommended to improve ADC accuracy?

3. What risks should we be aware of if lowering the impedance too far (e.g., signal distortion or increased power dissipation)?

Best regards,

Conor