Hello,

Does "Differential Input Threshold Voltage" change due to fluctuation of Vcc?

In the board suspected of fluctuation of Vcc, a plurality of points switching
from High to Low was confirmed.(For example, +3mV, +12mV, +25mV)

(Since Vcc is usually constant,
"Differential Input Threshold Voltage" is a fixed value between -0.2 V and +0.2 V.)

Regards,
Tatsu

Parents
• Hi Tatsu,

Typically the differential input threshold voltage is affected by the Vcc voltage, receiver common mode voltage and IC temperature. The threshold will also vary slightly between different devices. Across the full range of Vcc, Vcm and temperature, the differential input threshold voltage will be within the bounds of +0.2V and -0.2V.

Could you provide more details on how you are measuring the differential input threshold voltage? Did you measure this point when there was a DC differential voltage across pin A and B, or was it measured during an AC transition from logic high to logic low? Was the plurality of points measured on the same device?

Regards,

Neil

• Hi Neil-san,

When there is fluctuation of Vcc,
the point of switching from high to low is fluctuating,
but the point of switching from low to high is not fluctuating.

Is there any cause other than fluctuation of Vcc?
Or is this a normal operation of the ADM1485?

Regards,
Tatsu

• Hi Tatsu ,

If the switching point from low to high, and from high to low remains within the range of -200mV to +200mV, then this would be expected behavior from the ADM1485.

Just a tip, when trying to measure the receiver high to low and low to high trip points, adding 10nF capacitors between the A/B pins and GND can help filter out external noise on the input pins and give a better measurement of this receiver threshold. This is purely for your measurement purposes, these capacitors should not be present in the end application.

Kind Regards,

Neil

• Hi Neil-san、