Using an Op Amp to extend the voltage of a RF Detector?

[mrkenneth]

I would like to connect 8 Analog Devices AD8314 RF Detectors to a PIC16F877A microcontroller with 8 onboard 10-bit A/D convertors. The output voltages of the AD8314 RF Detectors are from 0V to around 1.2V DC, but 0.1V to 1.0V is sufficient for my use. The PIC will be powered from 5V, and since the A/D convertors are 10-bit, then each "step" would be 5V / 1024 = 4.88mV. I would like better precision, with each step around 2mV to 3mV. Is it possible to use an Op Amp to multiply the 0.1V to 1.0V range to a range of 0.5V to 5.0V? The AD8314 RF Detector outputs a fluctuating DC voltage. I have never read about an Op Amp operating with DC, so I am not sure whether this setup would be possible. If an Op Amp can be used in this application, should I be using a Precision Op Amp, with 4 Op Amps in one IC package? I do not have any experience with Op Amps, so any advice is greatly appreciated!

 

[Skogsgurra]

An opamp is the perfect choice in this case. They were designed for DC signals in the first place and do an amirable job with very small errors and great linearity.

You can use any 5 V single-supply opamp. The main concern is probably noise and offset voltage. A typical standard opamp will have offset voltage around a couple of mV, so your LSB will not be affected very much. The noise will be even less in a typical low-gain circuit.

There are several "cook-book" sites on the internet. Google for operational amplifier applications.

 

[groundhog1]

I am not an expert either, but look up Instrumentation amps. There may be something in that direction, although my memory is fuzzy (i haven't done the detector circuit for awhile). But I think it goes to DC if that is what you are after.

This is a configuration of several op-amps to make a better amp. You can also get them already made in one package, like an SO-8. Try Burr Brown or Analog Devices.
AD620 maybe??

groundhog1

 

[mrkenneth]

Thank you both for the great responses!

I have looked at some "Precision Op-Amps" at National (first company that comes to my mind with the Bob Pease shows). I narrowed down my choices to the LMC6484 and the LMC6084. They both offer a low offset of 0.75mV and 0.35mV with the more expensive "A" versions, respectively, which should be good enough. I should only need 2 ICs for 8 inputs/outputs. National offers an online simulation program that calculate all the resistor/capacitor values, so I do not have to do it myself. However, the simulation program requires a 7.5K-ohm resistor for Rg before the input of the op-amp.

Is that required? The AD8314 RF Detector can only source a maximum of 10 mA, so it should not damage the Op-Amp.
Is there anything else that I should consider?

Thank you in advance!

 

[Skogsgurra]

mrkenneth,

Yes. The Rg resistor sets the gain. So it is definitely needed. The Rb, however, is not strictly needed. Rb is used to balance out the bias current part of the offset and is usually chosen to be equal to Rg//Rf.

With the input bias current in the low low nA region, the Rb is not necessary. The difference with and without will be less than a microvolt - surely something you need not care about.

 

[Madcow]

If you have the pins and resources on the pic you can use
an external reference of 1.024v and get a 1mv resolution.

 

[mrkenneth]

Thanks skogsgurra! I actually meant Rb, not Rg. Sorry! So Rb would not be needed if I do not mind the small offset, right?

Would using +8V as the positive supply and +5V as the reference voltage work?

Unfortunately Madcow, I will have used up all eight A/D pins, so a lower external reference voltage is not possible. I am stuck with using Vdd and Vss (5V and ground) as the reference.

Thank you to everybody for the help!

 

[Skogsgurra]

mrkenneth,

Since the offset difference is in the microvolt region and your bits are worth millivolts, you do not have to worry.

Yes, you can use 8 V. The device has +15 V maximum supply voltage. So it is quite safe.

 

[mrkenneth]

Thank you skogsgurra for the confirmation!

 

[circuithacker]

Use a difference amplifier (AD620 or similar) set for a gain of 2 or so and amplify the difference between the AD8314's log (RSSI) output and ground. Then you won't have to worry about loading down the output of the AD8314 with resistors or funky level-shifting circuits.

You do want to ensure that the ADC in the PIC uses the power supply as its reference and that both the AD8314 and the PIC use the same supply if you want the measurements to be ratiometric (proportional to supply so that drift and voltage errors cancel).

"Fluctuating" is a misnomer; each of the detectors is a full-wave rectifier with a current output. The currents are summed and low-pass filtered via an on-board capacitor and you can add additional capacitance externally to lower the cutoff frequency of the LPF.

 

[mrkenneth]

Thank you for the informational reply circuithacker.

Unfortunately, the AD8314 will operate from a different voltage regulator. The ADC and PIC will use the same rail, however. The reason is for additional filtering of the power rail to the AD8314 and my wanting to operate the AD8314 at 3V (less current and perhaps longer life.) Will there be much error with this set-up? If the offsets are high (due to whatever reason,) I think I might just calibrate them and use software to correct the errors once everything is finalised.

Thanks for pointing out the incorrect use of "fluctuating."

 

[mrkenneth]

Another question:
Since I will be using using 8 op-amps, would it be okay to use one gain resistor (Rg) to set the gain for all eight op-amps? I will be using one supply rail for both Vref and V+ of all 8 op-amps.

Would it be better or worse than using one Rg resistor for each op-amp? I am using 5% carbon film resistors for now; should I use metal (oxide) film resistors instead?

Thanks in advance!

 

[mrkenneth]

I decided to use separate resistors for each op-amp now, because it is easier to route on a single-sided PCB.

I know this is a stupid question, but do all op-amps follow the same formula for non-inverting designs?
Vout = [(1 + Rf/Rg) x (Vin - Vref)] + Vref
If so, can I just use the calculator in National's website for op-amps from Linear and others?

Would 5% resistors be okay, or should I use something better?

 

[blcpro]

I believe the PIC you have chosed can operate at a VDD down to 2V, so you can still use the VDD reference voltage if you supply the PIC with a lower voltage. Just another possibility.

 

[mrkenneth]

I believe only the PIC16*L*F877A operates down to 2V. Unfortunately, the LF version only works up to 4 MHz. I would like the PIC to be 20 MHz.

I looked at other websites and found that the gain is just 1 + Rf/Rg. Much easier to remember and calculate than National's formula.