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Jan 10, 2018

Prototype Update

Hi all,

Happy new year!

I’ve been in Mexico for 2 weeks, but upon return I assembled (well, channels 2+3 at least) the latest bare PCB from AllPCB.com that was waiting for me in the mail. And after a couple tweaks, I think I finally have a board ready for production! Here it is:

Now that I’m happy with it, I’ve ordered 4 final prototypes from MacroFab.com, fully assembled. They’ll arrive at the end of the month. Once I’ve tested them, I’ll make a few more minor tweaks, if needed, and then order 200 boards, which should arrive towards the end of February. So you should start getting your orders in early March! (Mugs will ship out now.)

Some new features:

  • Even less noise! The analog parts (ADC, op-amp, etc.) are now powered from a separate voltage regulator, providing some needed isolation from the noisy digital (FPGA, etc.) current draws. I’ve also added extra low-pass filtering to the DAC offsets to the op-amp.
  • The USB2 optional add-on board now fits better than ever, like they were meant to be together. (That’s the purple board in the pic above.)
  • Adjustable frequency compensation. What’s that? Basically, it gives you two little knobs to turn on the board for each gain of each channel, that lets you match the gain at high frequency to that at low frequency. It makes a square wave square! (You have these on oscilloscope probes too.) If you want to know more about it, read below!

I’ve also received all the other parts that will go in the Haasoscope bundle, etc. Here’s what (some of) 400 oscilloscope probes looks like!

The Story of Compensation:

You may know about the basic op-amp negative feedback gain circuit:

The gain is given by -R_f / R_in. However, when R_in is big (and for us it’s 1MOhm… you want high impedance in an oscilloscope, to not significantly affect (load) the signal you’re trying to measure!), there can be smaller impedances at large frequency. Remember than impedance for a capacitor is 1/wC, where w=2pi*f. At 1 MHz, even a “stray capacitance” of 1 pF gives an impedance of 1/2pi MOhm, or ~150k! And there’s stray capacitance everywhere, between any two wires, it’s unavoidable! The gain is then no longer controlled by R_f / R_in, it’s dominated by C_in / C_f !

So what do we do? The trick is to add some additional capacitance intentionally, that you can carefully control. So instead of C_in and C_f being stray capacitance, it’s a reasonably solid value. As long as C_in / C_f = R_f / R_in , the gain is always the same, at low or high frequency! Here’s a little more info about it: https://electrosome.com/compensated-attenuator/ . You may have noticed on your big old oscilloscope that the inputs are “1 MOhm, 13 pF”, well, this is where that 13 pF comes from and why it’s there! (Also so 10x probes are easy to use/make… but that’s another story!)

For example, for “high” gain, we want a gain of 2. I use a 2MOhm R_f (actually 2 1MOhm resistors in series, to save $!), and R_in is 1 MOhm. Then I add an intentional capacitance of 10pF in parallel (C_in), so I want C_f to be 5pF. Great, done! Well, almost. There’s still about 1 pF of stray capacitance, so the ratio of C_in/C_f is not going to be exactly 2, unless you get lucky. You need to adjust the capacitance (within a few pF), to account for wherever that stray capacitance happens to be coming from. It can also change with temperature, humidity, component ages, etc.! So that’s what we do - we add little adjustable capacitors to the board, to let you fine-tune C_in/C_f, so the gain is constant vs. frequency. You just need a little tiny flat-head screwdriver (plastic, so you don’t add capacitance from your body while adjusting it!). I’ll tune them all before I ship it to you, but you may need to adjust it later. This is what it finally looks like (for all gains and all channels):

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Now we're talking about a serious data-acquisition system - 4 Haasoscopes!

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USB-serial Adapter

This plugs into your computer (USB port) and then connects to the right side of the Haasoscope (serial RX/TX). It also can power the Haasoscope. You need one of these if your computer doesn't have a serial port, which is almost all modern computers. Arduinos and Raspberry Pi's do talk serial though - you can use them to read out the Haasoscope without an adapter!



This lets you reprogram the Haasoscope FPGA firmware from either Windows or Linux using the free Altera Quartus II software via the JTAG connector.


A Cool Screen!

This is a 0.96" 128X64 pixel white OLED screen. It communicates with the Haasoscope over an SPI interface, and can show ADC data from a selected channel, or whatever you tell it to! It can plug directly into the header above the FPGA.


Oscilloscope Probes

Two passive 100 MHz bandwidth oscilloscope probes for connecting to Haasoscope 100 MHz ADC inputs using BNC.


Opto-isolated USB-serial Adapter

For those of you probing some dangerous stuff, you can now act crazy without killing your laptop. This USB-serial adapter works just like the standard one, but is opto-isolated, for 1500V of protection!


High-speed USB Readout Board

In case 1.5 Mb/s is not enough bandwidth for you, grab one of these boards and have high-speed USB2 output from a Haasoscope! Using just 8+2 digital outputs on the Haasoscope, you can get about 4 MB/s, about 20x faster than serial, and still use the same python readout code. It's supported in the stock firmware too! Using 8+4 digital outputs, you could in theory even get up to 40 MB/s using C++ readout code and the free FTDI USB libraries.


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Andy Haas

I teach physics at NYU. I use electronics for research, in teaching, and as a hobby.

Andrew Haas

Seeed Studio

PCBA Manufacturer

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