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?

  • 1
    \$\begingroup\$ 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 \$\endgroup\$ – jsotola Feb 25 '18 at 7:13
  • \$\begingroup\$ @jsotola Makes sense. There is a spec about minimum common mode voltage in the datasheet though - I assume that has to be met also? \$\endgroup\$ – Alex I Feb 25 '18 at 7:15
  • \$\begingroup\$ 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 \$\endgroup\$ – jsotola Feb 25 '18 at 7:20
  • \$\begingroup\$ 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 \$\endgroup\$ – jsotola Feb 25 '18 at 7:22
  • \$\begingroup\$ What chip is it? \$\endgroup\$ – Andy aka Feb 25 '18 at 9:59

The differential input voltage will have a nominal average signal level of 1.2 V.

To supply a constant value, just drop a 100 Ω resistor across each pair, and source 12 mA through each. Make sure you add an additional 100 Ω before the first pair, and after the last so that no input is connected directly to VCC or ground.

This is from the TI SN65LVDS4 datasheet (the receiver I typically use).

signal levels

One of the most common problems with differential signaling applications is how the system responds when no differential voltage is present on the signal pair. The LVDS receiver is like most differential line receivers, in that the output logic state can be indeterminate when the differential input voltage is between –50 mV and 50 mV and within its recommended input common-mode voltage range. Open circuit means that there is little or no input current to the receiver from the data line itself. This could be when the driver is in a high-impedance state or the cable is disconnected. When this occurs, TI recommends to have an external failsafe solution as shown in Figure 20. In the external failsafe solution, the A side is pulled to VCC via a weak pullup resistor and the B side is pulled down via a weak pulldown resistor. This creates a voltage offset and prevents the receiver from switching based on noise.

signal test


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