# Logic Level Converter for Negative Voltages

Here is logic level converter mostly used for converting signals between 3.3V and 5V. (Other positive levels such as 1.8V are also possible). What if I want to convert from 3.3V to -5V logic (negative!)? It doesn't have to be bi-directional. Could we use a P MOSFET instead? And how the schematics would be?

NMOS example for positive level converter:

• What are you connecting to that requires -5 V logic? Are you sure you don't mean inverted logic where a '1' becomes a '0' and vice-versa? Write out a proper specification table. (Hint: "Other positive levels are also possible" isn't adequate.) Mar 1, 2020 at 17:21
• Nuclear Instrumentation Module (NIM) has 2 logic-level-ranges called slow and fast. The fast NIM logic has -1.8V and -0.8V for logic 1 and 0 respectively. I have TTL signals to be converted to this logic. Mar 1, 2020 at 18:05

simulate this circuit – Schematic created using CircuitLab

Given a 0V/+3.3V digital input signal (blue), the output (orange) will be:

Analog devices make digital isolation products such as the ADuM260. There is a whole range of devices you can pick from with various differences such as IO direction, mixed direction IO etc. and are fully isolated to several kV hence, going between positive supply logic circuits and negative ones is straightforward. Some devices even have built in power converters such as the ADuM5410.

• How about just 1 signal to be converted. In that case this would be unnecessarily big. Mar 1, 2020 at 18:07
• It’s still a lot smaller than the four channel device you linked in your question so please don’t grumble about this and, it comes ready with a data sheet telling you how to use it in your type of application. Mar 1, 2020 at 18:11
• I was talking about only one channel of it which is just 1 n-type MOSFET and 2 resistors. Mar 1, 2020 at 19:03
• Good luck with that then. Mar 1, 2020 at 20:00

You could also make use of the common-mode voltage range of an RS-422/485 line receiver. Power the receiver with Vcc=GND, and GND=-5V (or -3V, depending on the device), and connect one of the diff inputs to a voltage divider to provide a threshold of ~+1.5V.

We've used the old Harris HS-26C32 for just such a purpose. Just need to be make sure you pay attention to the common mode voltage range of the chosen receiver and the Voh output levels of whatever is going to driver the receiver.