We use cookies to provide our visitors with an optimal site experience. View our privacy notice and cookie notice to learn more about how we use cookies and how to manage your settings. By proceeding on our website you consent to the use of cookies.
I bought a HCNR200 linear optocoupler on DigiKey, but it doesn’t seem to work properly, I input a signal from 1kHz to 20kHz. I connected an oscilloscope to the LED of the optocoupler, the signal is fed to the LED linear, and from the output of the optocoupler on the photodiode, I get a signal that goes down with increasing frequency. It looks like I bought a fake chip from you. Is that possible? Or did I do something wrong?
The amplifier is an opa827 transimpedance opa827, also purchased from you. I looked at the linearity on the LED, the signal is confidently held up to 200 kHz, but at the output signal with increasing frequency drops, does not hold the achh, I measured the signal and from the photodiode, and from the photodiode connected through the amplifier transimpedance.
I soldered according to this diagram from the datasheet, resistors Chinese, transistor 2N5401 instead of 2N3906, Capacitors 47 picofarad and 33 picofarad reduced to 10 picofarad to not reduce the frequency range, and also removed 47 picofarad completely and instead of it put a large resistor. Oscilloscope touched the LED leg, and on it the linear signal is given in a large frequency range, 200 kHz linearly, and at the output on the photodiode, the signal is reduced and even this is not good for 20 kHz, and also from the opa380 amplifier measured, also a strong dropout at 20 kHz. And the datasheet says - Wide Bandwidth - DC to >1 MHz
The HCNR200 contains a LED and two photodiodes. Linearity is increased by wrapping one of the diodes into a feedback loop. For your schematic:
LED is the emitter for the optocoupler
PD1 is the sensing (feedback) diode
PD2 is the output photodiode
Note that this project is adjacent to a 2018 blog post.
Thoughts:
The blog posts states " It only work with positive input voltages (VIN ≥ 0) regardless of single or dual supply of ap-amp." This may reflect the indented 4-20 mA industrial applications typically associated with the HCNR200.
Eliminate the Q1 and D1. The op amp should be able to drive the LED directly as suggested in Application Note 5394 (Avago) as shown in the schematic.
Note that the current through PD1 and PD2 should be identical. Note the values of R1 and R5 in the schematic below.
When using BNC we are in a 50 Ω system. Be sure to provide a termination resistor directly to ground at the input BNC connector. Reflections in the BNC cable can cause problems at higher frequencies.
Please provide the schematic for your circuit as the blog post diagram if very difficult to read as it does not follow a conventional left to right flow.
With regards to frequency:
Since PD1 and PD2 are identical, you could measure the frequency dependent voltage on PD1 and compare it to PD2. It should be the same.
Assuming a non-linear frequency response (but compensated by the PD1 feedback loop) verify that the PD1 feedback signal isn’t “in the rails.”
Please let us know if you were able to achieve your high frequency requirements.
I do not see any evaluation boards in our catalog that match the same isolator at this time. Would you be okay with a potential alternate, or do you need to work with this exact IC?
I would like the linear optocoupler to be high-frequency and preferably not worse in characteristics than HCNR201. If there are linear optocouplers with better characteristics than HCNR201, I will buy them. I need to work in frequencies from 20 Hz to 20 kHz, but it will be better if from 1 Hz to 100 kHz and higher.
Searching for Optocouplers that have a Photovoltaic, Linearized output, I found some rated for a higher frequency. Click here for these options.
Please check the datasheets to verify compatibility to your application.
