Newport Optical Powermeter
I’ve previous written about my two other optical power meters, and compared their performance in the first of these articles. Today, I’m going to take a quick look inside the most expensive of my meters, the Newport 841-P-USB. I’ve written up my notes on actually using it previously. So now lets take a look inside:
The main microcontroller seems to be a MSC1210. This is an 8051 microcontroller with an 8 channel ADC. Over near the sensor connector there’s an AD8571 opamp and on the other side a ADG709 analog switch.
This meter can accept a number of different heads depending on power range and accuracy requirements. These would either be a photodiode, or thermopiles heads.
For lower powers a photo diode head would be used. Typically the readout from these heads would be a small (nanoamp) current.
The AD8571 is a low input bias current part, I assume this would be suitable for use as a transimpedance amplifier sensing nanoamp currents from photodiodes. Looks like this is connected to a 50K feedback resistor. This would mean 100nA would result in an output of 5mV. Which seems kind of low, but in the right ballpark.
Elsewhere there’s a OP2177 this seems more typical of precision voltage gain circuits I’ve seen. Thermopiles are essentially set of thermocouples in series. So, I’d expect a voltage gain configuration here. Perhaps this is what they’re using the OP2177 for.
The ADG709 could be being used for switching feedback resistors, or possibly they’re using the large relays for this.
On the other side of the board we find the PL-2303. This is a USB Serial IC. The meters serial interface is really really basic. It feels like there was possibly an old 8051 serial interface design which they slapped a USB interface on.
The head itself is difficult to non-destructively disassemble, but we can take a quick look:
You can remove the top of the head, making the surface a little more visible. The surface is coated to aid the adsorption of the light (to help the thermopile heat up thereby measuring the optical power). This is also why the head has a large heatsink and fan. The head is essentially measuring heat as a proxy for optical power, so has a tendency to get rather hot.
That’s it for today! This was the last of the optical power meters I’ve actually used but at least one more knocking around which will be featured in a future post.
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