Using an OpAmp... Help! - Page 1
Forum section: electronics / beginners

author: edy
author_rank: Super Contributor
timestamp: January 08, 2013, 04:39:22 pm
content: Hi folks,I am working on a pulse ox project and managed to get my hands on an ADA4817-1ACPZ-R7 "Low Noise, 1 GHz FastFET Op Amp" to use for reading the output from the phototransistor sensor, before I capture it using the ADC on my Arduino. The package is TINY and I will have to get a PCB to solder it to so I can break the pins out to a breadboard.Here the datasheet:http://www.analog.com/static/imported-files/data_sheets/ADA4817-1_4817-2.pdfHere's the pulse-ox basic layout:I am a bit puzzled as to what I am going to do with this chip since there are more pins on it than I was prepared to hook-up. Any help would be appreciated knowing where to begin! Thanks so much.

author: edy
author_rank: Super Contributor
timestamp: January 09, 2013, 04:02:13 am
content: Hi Folks,I received another couple of devices today. One is this OpAmp "ADA4004-1ARJZ-R7" (specsheet here):http://www.analog.com/static/imported-files/data_sheets/ADA4004-1_4004-2_4004-4.pdfAnd another is "AD8033AKSZ-REEL7" (as shown here):http://www.analog.com/static/imported-files/data_sheets/AD8033_8034.pdfI have the 5-lead packages for both, with pins "OUT", "+V", "-V", "+IN" and "-IN". From what I read, an op-amp is a differential amplifier which outputs voltages thousands of times greater than the difference between the input voltages. Depending on the application, it can be used for many different purposes.I am trying to wrap my head around these and how I can use them for my purposes in building a pulse oximeter. The sensor LED (red/infrared) can be driven using an Arduino by using 2 pins to alternate between the HIGH/LOW, with the LED configured in opposite directions. While this is going on, I have a phototransistor picking up the signal from the LED's and it is this current which is important to detect.Therefore, the op-amp can be used to pick up small changes in the phototransistor/diode and multiply them significantly so I can pick up analog voltage difference in my Arduino. I see by the schematic that it is being used as a "Trans-Impedance Amp" so I found this interesting article:http://electronicdesign.com/analog/whats-all-transimpedance-amplifier-stuff-anyhow-part-1So if I design this into my Arduino-based pulse-oximeter, I am using it as a current to voltage converter as follows:http://en.wikipedia.org/wiki/Current-to-voltage_converterBecause the sensor in the pulse-ox probe is a phototransistor which varies current based on LED light falling on it. I need a way to detect the voltage into my analog voltage inputs on the Arduino so using the op-amp will produce a change in voltage as this diagram illustrates:So I stumbled upon this along the way: "Circuit Fantasia"http://www.circuit-fantasia.com/I'm still trying to understand this device and what values I need to use for my resistors to get a range of voltages that I can use for proper useful inputs to analyze the signal to determine the oxygen ratios. Argh!

author: user_ivo
author_rank: Contributor
timestamp: January 09, 2013, 10:33:23 am
content: If i undertood, you want to build something like this (See the attachment)Specify the type of diode that you are thinking to use and the maximum output voltage that you can apply in the arduino input (ADC input), so in that way you can find the value of R.Vou_max= Iphoto_max * R

author: edy
author_rank: Super Contributor
timestamp: January 10, 2013, 06:39:48 pm
content: Yes, thank you. That's it. I see the inverted input (-) and normal input (+) coming into the op-amp. I see the "+" input goes to ground of Arduino, as would the ground of the phototransistor. The other end of the phototransitor connects to the "-" of the op-amp, and there is a resistor feedback connecting Vout to the "-" input which I need to decide on the "R" value based on your equation.There is also VCC+ and VCC- labeled on the op-amp which is the "helping" voltage source. I am trying to figure out how to wire that part up to my Arduino. I assume the +VCC goes to the +5V lead on my Arduino, but where do I connect the -VCC as I assume it is not to ground, but a -5? Wouldn't that make the total voltage difference 10v? Or am I confused again here?

author: user_ivo
author_rank: Contributor
timestamp: January 11, 2013, 11:56:08 am
content: That´s it, if you read in op-amp datasheet his say that the op-amp could operate in single supply, so you can connect the -Vs to arduino ground and +Vs to +5V lead of arduino. Your output max voltage in this case will be 5 V. normal is usual connect a bypass capacitor [1-10uF] between the Vs lead and the Ground ( see the example in datasheet).Note the cathode of photo diode must be connect to ground and anode to the inverted output of op-amp, in this case your output will be positive, in opposite case the direction of current will be different and the output in this will be negative ( or zero if you connect the -Vs to ground instead of -5 V ).If you have further question let me now.