# How to send logic low to LVDS?

I have a chip that takes LVDS inputs. It has a very large number of inputs and I'd like to set most of them to zero (or one) while prototyping. They can't be left open/floating.

What is a good way to send a fixed LVDS logic value to a bunch of pin pairs? (probably using passives, or a driver that can drive multiple differential pins at the same time)

The chip uses LVDS1.8, the datasheet looks like this:

Parameter                                         | Min | Typ | Max  | Unit
Input Common-Mode Voltage Range, VCOM             | 825 |     | 1575 | mV |
Logic High Differential Input Threshold, VIH_DTH  | 175 |     | 400  | mV |
Logic Low Differential Input Threshold, VIL_DTH   |−175 |     |−400  | mV |
Receiver Differential Input Impedance, RIN        |  80 |     | 120  | Ω  |
Input Capacitance                                 |     | 1.2 |      | pF |
LVDS Input Rate                                   |1250 |     |      |MSPS|
LVDS Minimum Data Valid Period                    |     |     |  344 | ps |


Can I just use two adjustable supplies set to 1200mV-200mV and 1200mV+200mV, and connect all the P pins together to the high voltage and the N pins to the low voltage? What current would I expect per pin pair?

• you only need to supply 350mV across the input pins. ... the polarity determines the logic value (0 or 1) ... normally the voltage is generated by a resistor that has current flowing through it, supplied by a constant current source. ... if you are not using cables during prototyping, and since you want to supply a steady-state value, then you should be able to supply all the inputs in parallel by using 350mV power supply .... en.wikipedia.org/wiki/Low-voltage_differential_signaling Commented Feb 25, 2018 at 7:13
• @jsotola Makes sense. There is a spec about minimum common mode voltage in the datasheet though - I assume that has to be met also? Commented Feb 25, 2018 at 7:15
• whatever one input needs, gets fed to all the inputs ... you are not feeding in any AC signals, only a steady state logic level .... same thing as using pullup or pulldown resistors in other logic circuits Commented Feb 25, 2018 at 7:20
• it is possible that you could use a constant current source and daisy-chain the inputs (connect them in series) ... but i have no idea if that would actually work Commented Feb 25, 2018 at 7:22
• What chip is it? Commented Feb 25, 2018 at 9:59