Sensor Interface Question(s)
Sensor Interface Question(s)
(OP)
Hi,
I need some help.
I'm interfacing an analogue sensors to a 5 volt dc micro.
The sensors have a mv output, 0 to 100mv & 0 to 2.5v
I need to scale the signals to 0 to 5v dc and i've been told that i need a high impedance input.
How do i do this? I have 5, 10 & 12 vdc availiable.
I also have to convert:
12v dc (on or off) to 5vdc (datalogger input is pulled high by 100K)
24v dc (on or off) to 5vdc (datalogger input is pulled high by 100K)
Any assistance you can give would be great. Assume I know nothing!
Dansoarr
I need some help.
I'm interfacing an analogue sensors to a 5 volt dc micro.
The sensors have a mv output, 0 to 100mv & 0 to 2.5v
I need to scale the signals to 0 to 5v dc and i've been told that i need a high impedance input.
How do i do this? I have 5, 10 & 12 vdc availiable.
I also have to convert:
12v dc (on or off) to 5vdc (datalogger input is pulled high by 100K)
24v dc (on or off) to 5vdc (datalogger input is pulled high by 100K)
Any assistance you can give would be great. Assume I know nothing!
Dansoarr





RE: Sensor Interface Question(s)
To get 0 to 100mV to 0 to 5 volts you need a gain of about 50 with the op-amp. Select the resistors for a gain of 50.
Now, what I posted above is the simple case. In actually, even rail-rail op-amps don't quite reach the rails, and this problem becomes more apparent the higher the gain. Also, "0" input sensors may not really be zero, and micros may not be able to digitize all the way to their 5 volt rail. Inputting a little offset adds another op-amp, and possible a reference into the circuit - that is if you need to zero-out any offsets or you need high accuracy.
Also, if your sensors are real high impedance, you may have to use a different op-amp gain approach, or buffer the signal before the op-amp.
For your 12 volt and 24 volt digital input signals, put a series resistor (20K ohms) to a 4.7 volt zener cathode with the anode connected to ground. Across the zener place another resistor (say 10K). Zener limits to about 4.7 volts max. The series resistor limits current to the zener. The resistor across the zener pulls the signal to ground (actually about 0.5 volt) if the input is open.
I've just put general info here. Don't have time for more detail post at this time. Questions?
RE: Sensor Interface Question(s)
RE: Sensor Interface Question(s)
RE: Sensor Interface Question(s)
A better way is to use a non-inverting configuration. Formula for gain is almost the same, just add 1. Like this: Gain = 1 + Rfb/Rin.
That means 100 kohm for Rin and 100 kohms for Rfb at gain 2 and Rfb = 4900 kohms for gain 50. I would use lower valued resistors, though.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Sensor Interface Question(s)
on the 12 & 24 volt to 5 volt. I'm assuming that for a vf of 1.25v and if=2 mA, id need a 11k resistor for 24v and 5.1k for 12 v.
my input signal would go to the resistor then to pin 1, through the led then out of pin 2 to ground.
Pin 4 would be connected to ground, pin 5 connected to vcc via the 100k pullup on the micro. do i leave pin 6 (the base) not connected ?
when serching for the 4n35, i came across this device at www.avagotech.com hcnr200, described as an analogue optocoupler would this work for my sensor?
thanks
RE: Sensor Interface Question(s)
RE: Sensor Interface Question(s)
To answer your question on the HCNR200 - When using an opto in an analog application you typically use part of the opto in a feedback circuit to linearize the response. It's not as simple as just passing the signal through the opto.