---
title: "W5100S EVB Pico ADC floating values when input sensor is disconnected"
url: "https://maker.wiznet.io/forum/15956"
markdown_url: "https://maker.wiznet.io/forum/15956/md"
type: "Forum topic"
category: "MCU & Boards"
author: "kkshah73"
created: "2025-12-18T20:14:18+09:00"
last_activity: "2025-12-19T09:12:44+09:00"
language: "en"
tags: ["MicroPython"]
views: 5
replies: 2
source: "WIZnet Makers forum (https://maker.wiznet.io/forum/)"
---

# W5100S EVB Pico ADC floating values when input sensor is disconnected

## Question

Asked by kkshah73 on 2025-12-18 in MCU & Boards.

Hello

I am using a W5100S EVB Pico board and reading sensor using the RP2040 ADC (GPIO26).

When the sensor is connected, the ADC readings work correctly. However, when the sensor is disconnected, the ADC pin becomes floating and returns random / noisy values, which results in false readings.

Setup Details

Board: W5100S EVB Pico

MCU: RP2040

ADC input: GPIO26

Sensor: 100A/ 1V DC - CT (AC current measurement)

Language: MicroPython

Ethernet: W5100S

Issue

ADC values are stable with CT connected

ADC values become random when CT is unplugged

Need a reliable way to ignore or detect floating ADC input.

Questions

1. What is the recommended hardware approach on W5100S EVB Pico to prevent ADC floating when an analog sensor is disconnected?

Pull-down or pull-up resistor value?

Biasing best practices?

2. Are there any WIZnet reference designs or recommendations for handling disconnected analog sensors on RP2040?

3. What is the preferred software method to detect a disconnected CT sensor (RMS threshold, variance check, etc.)?

Any guidance or example circuits/code would be greatly appreciated.

Thank you.

## Replies

### Reply 1 by kkshah73, 2025-12-18

There is any way to handle by software only? I don't want add external component for that.

### Reply 2 by Alan, 2025-12-19 (in reply to reply 1)

It would be best to resolve the RP2040 issue through the Raspberry Pi forum.
However, in my opinion, this issue can likely be addressed at the firmware level by applying a pull-down.
Thank you.

---

Source: https://maker.wiznet.io/forum/15956