| Electronics > Projects, Designs, and Technical Stuff |
| DIY Modular Test Equipment Project |
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| void_error:
After spinning around in circles looking for suitable DACs with an LDAC pin in order to update two power supply or DC load channels at the same time it seems I'm back to the initial selection of cheap string DACs. --- Quote from: void_error on November 10, 2016, 11:54:25 pm --- * DAC * 16bit * AD5662 * DAC8501 * DAC8531 * DAC8551 --- End quote --- None of them have a LDAC pin which means I'll need to find a way to update them at the same time. Having the MSB the first bit that needs to be shifted in makes it more difficult so I should find a way to delay the LSB by at least one clock cycle and use that to clock it in simultaneously for both DACs. Some glue logic will be required or just ditch them and go for an AD5761 which has everything I need. The latter option is easier to implement so I'll probably be sticking with it. EDIT: Looks like the AD5761 is a clear winner. Just had a look at the waveform generator output stage and I'll use the 12-bit variant there for output offset adjustment since it gets rid of one op amp used for level shifting. Problem solved. |
| void_error:
Time for another short progress update... The UI board is going to use 3.3V logic, mostly due to the fact that the display is a 3.3V part and there's no information in the controller's datasheet regarding the output current of the data lines which means using open drain outputs and pull-up resistors to do the level shifting from the previously 5V powered MCU might not work properly so I'm not risking it. The changes to the UI board aren't going to affect the DC load or the Lab Power supply since they use digital isolators. What needs changing so far is the Waveform Generator / Frequency counter board and the good thing here is that the AD9834 DDS chip can use different supplies for the analog and digital supply pins which means I can get rid of a few parts like the level translator chips for the clock generator chips which run on 3.3V. The analog stuff will still run on 5V. Regarding the 595 shift registers, each board is going to use two of them because I couldn't find a way to implement all the SPI addressing and any other functionality using only 8 pins. I want to have everything ready for the PCB design stage by the end of the year but I might not be able to since the front panel board for the Lab Power Supply and DC Load as well as the digital channel for Lab Power Supply haven't been designed yet. |
| void_error:
Looks like the frequency counter part is almost complete with only a few tweaks here and there still to be done. Managed to get rid of the MCU and replaced it with about two truckloads of logic chips, mostly counters, flip-flops and multiplexers. It should go up to 20MHz @ 1Hz resolution with LVC series counters (74LVC161) but I've also included a divide by 10 prescaler which should allow it to go up to 100+ MHz with 10Hz resolution. |
| Kleinstein:
--- Quote from: void_error on December 31, 2017, 02:07:12 pm ---Looks like the frequency counter part is almost complete with only a few tweaks here and there still to be done. Managed to get rid of the MCU and replaced it with about two truckloads of logic chips, mostly counters, flip-flops and multiplexers. It should go up to 20MHz @ 1Hz resolution with LVC series counters (74LVC161) but I've also included a divide by 10 prescaler which should allow it to go up to 100+ MHz with 10Hz resolution. --- End quote --- This direction is odd. The more normal way today is to get rid of the logic chips and use a µC to implement a reciprocal counter. This way a frequency counter is a rather simple task and still can get really good resolution. So more like 8 digits of resolution after 1 or 2 second in a frequency range of about 10 Hz to around 50 MHz (more with prescaler) with just a single µC (like STM32F3...) and an external clock (e.g. TCXO) and the input stage. So the difficult part today are more like the input stage and maybe the software. So a frequency counter today is a small project, unless you want to go beyond something like 8 or 9 digts. With some special TDC modules / chips very high resolution is possible, that might want a good OCXO or more to get a meaning to 12 digits. |
| void_error:
--- Quote from: Kleinstein on December 31, 2017, 03:26:46 pm ---This direction is odd. The more normal way today is to get rid of the logic chips and use a µC to implement a reciprocal counter. This way a frequency counter is a rather simple task and still can get really good resolution. So more like 8 digits of resolution after 1 or 2 second in a frequency range of about 10 Hz to around 50 MHz (more with prescaler) with just a single µC (like STM32F3...) and an external clock (e.g. TCXO) and the input stage. So the difficult part today are more like the input stage and maybe the software. So a frequency counter today is a small project, unless you want to go beyond something like 8 or 9 digts. With some special TDC modules / chips very high resolution is possible, that might want a good OCXO or more to get a meaning to 12 digits. --- End quote --- Yes, I know it's odd. Initially I went for a local MCU as a frequency counter but I figured it wouldn't be too nice to force anyone who wants to build this to use a PIC (16F1619 in this case). But in one of the next revisions I will change to a MCU which is going to be installed on a daughter board so one can use their MCU of choice. With that in mind, the old-school version of the frequency counter it will be on a separate board, with the waveform generator having a MCU-based frequency counter. I'll build both versions, I'm curious how they perform. |
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