I have ordered this device AD8318, the datasheet can be seen here, http://www.analog.com/media/en/technical-documentation/data-sheets/AD8318.pdf And, the actual module using this device, and ordered, shown here: https://www.ebay.com/itm/1PC-AD8318-RF-1-8000MHz-logarithmic-detector-70dB-RSSI-measurement-power-meter/142140981389?ssPageName=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649

What I would like to do it use an LM3914 bar graph and show the RSSI in the series of 10 leds, which would provide the signal strength of the rf field it detected.

If I am reading this data sheet correctly, it produces a positive voltage on the output pin of 2.1V when signal strength is at its' LOWEST, and a 0.6V output at the HIGHEST detection signal it is capable of.

I would simply like to invert that, having the 2.1V when detecting the HIGHEST signal possible, and the 0.6V when detecting the LOWEST signal possible. Even better would be the capability to make that output between 0.0V and 1.5V.

 

You can invert the signal with an inverter op amp circuit.
You need to also input a bias voltage to offset it to a positive output.

The LTspice simulation of an example circuit is shown below.
Pot U2 controls the offset and pot U3 controls the gain. There is some interaction between the two so you may have to go back and forth between them to get the exact output you want.
The pots can be 10-turn trimpots.
Note that it uses a rail-rail output type op amp.

 

You can invert the signal with an inverter op amp circuit.
You need to also input a bias voltage to offset it to a positive output.

The LTspice simulation of an example circuit is shown below.
Pot U2 controls the offset and pot U3 controls the gain. There is some interaction between the two so you may have to go back and forth between them to get the exact output you want.
The pots can be 10-turn trimpots.
Note that it uses a rail-rail output type op amp.

View attachment 145005

Thank you, and for such a fast response. I will look through the junk parts drawer and see what I have which might work for that opamp, and if I have suitable 10-turn pots, I am worried I don't have the 20k, but do have 10k and 50k. But, must check first, and will have time in a few hours.

AGAIN. Thank you, now I can think about this and go over in the mind and prepare for the build.

 

The op amp has to be a rail-to-rail type to operate from a single 5V supply.

You can use other pot values.
Just change the associated resistor values proportionally.

 

You can invert the signal with an inverter op amp circuit.
You need to also input a bias voltage to offset it to a positive output.

The LTspice simulation of an example circuit is shown below.
Pot U2 controls the offset and pot U3 controls the gain. There is some interaction between the two so you may have to go back and forth between them to get the exact output you want.
The pots can be 10-turn trimpots.
Note that it uses a rail-rail output type op amp.

View attachment 145005

Would it be possible to just slap a ~.6v forward biased diode from the non-inverting pin of the opamp to ground, and forward bias it with the 37.4k resistor and just set a fixed 0.6V at the non-inverting pin? I have 10k pots, I am sure, so can I just series a 10k fixed resistor and 10k pot, limiting my range of adjustment of gain, but probably still in the range I would be using?

Or, could I just bias the lm3914 and led bar graph above the ground which the ad8318 modules sees, with a diode, with suitable bypass cap, setting the ground for the lm3914 ~0.6V above ground? And, the small amount of offset error just ignored in operation?

 

Would it be possible to just slap a ~.6v forward biased diode from the non-inverting pin of the opamp to ground, and forward bias it with the 37.4k resistor and just set a fixed 0.6V at the non-inverting pin?

Possible, but diodes have a significant temperature coefficient.

I am sure, so can I just series a 10k fixed resistor and 10k pot, limiting my range of adjustment of gain, but probably still in the range I would be using?

If you use a 10kΩ pot for offset at the plus input, then use an 18.7k to the +5V.

Or, could I just bias the lm3914 and led bar graph above the ground which the ad8318 modules sees, with a diode, with suitable bypass cap, setting the ground for the lm3914 ~0.6V above ground? And, the small amount of offset error just ignored in operation?

But that would give the maximum bar graph at the minimum signal.

 

Here's the circuit with 10kΩ pots: