Voltage divider dropping voltage more than expected

Question

I'm trying to connect an ACS712 current sensor to an ESP32 ADC pin. The ESP32 expects 3.3 V, but the ACS712 can reach 4.5 V.

When there's no current going through the ACS712, its signal is 2.5 V, so I made a test: I made a voltage divider using 330 Ω / 1 kΩ resistors and instead of the 2.5 V dropping to 1.88 V as expected, it dropped to something like 0.4 V.

As soon as I connect the 1 kΩ between signal and ground, the signal drops from 2.5 V to 0.4 V.

Using much bigger resistors (4.7 kΩ / 10 kΩ) it works as expected, but I'm left wondering why it doesn't with smaller resistors.

Answer 1

The ACS712 datasheet tells us the device will source 3mA. 2.5V into 1k equals 2.5mA. That suggests you are loading it too much. 10k is 1/10th of the load, so the ACS712 is a lot happier.

Answer 2

The datasheet says the smallest resistive load is 4.7 kohms. Your resistance values are much lower than the allowed load, so the chip output can't drive so low resistance load.