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| RTC with 1 Hz output as default (no serial communication wanted) |
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| eV1Te:
I am currently designing an analog high impedance amplifier circuit, where i need to generate a built-in calibration signal that goes through all the amps stages to verify that the gains etc. are correctly set. The calibration signal will be connected to the input of the main electrometer-type amplifier (LMP7721) via a high-insulation-resistance reed-relay with guarded internal screen. For this application it makes sense to have a calibration square-wave of very small amplitude and a low frequency of ca 1 Hz. The amplitude will be handled separately by an additional op-amp stage. Now to my question: How to generate a reasonably accurate 1 Hz square wave? 1. Crystal with RTC IC is probably best... But without a micro I need a RTC chip that has 1 Hz output enabled by default. Is there any small (8-pin) RTC ICs that do that? In all chips I have seen its disabled by default, also enable/reset pin is desirable so it can be turned off when not used, for reduced noise on power-rails but it's probably not required. 2. Schmitt trigger oscillator or 555 timer comes to mind, but for 1 Hz you need large caps, you might not get 50 % duty cycle, and it might inaccurate/drift which could be confused by error or low quality by the end user. 3. A micro with crystal (or built in RC) would also work, but the extra complexity of having to write software and add programming headers to the PCB is not wanted. 4. Are there better ways? Thanks |
| schmitt trigger:
Probably your best bet would be to hack an “analog” clock controlled by a 32768 Hz tuning fork crystal. These devices output a bi-phase 1 Hz signal, and work from a single 1.5 volt battery. If you can find the raw IC, the part number is MC14450, although it may be obsolete. |
| blackdog:
Hi eV1Te, Use a cheap Arduino klok module with a DS3231 IC on it. Program one of the outputs to 1Hz, and i think one of the outputs is standard 1Hz, its a open source output. https://www.aliexpress.com/item/DS3231-AT24C32-IIC-High-Precision-RTC-Module-Clock-Timer-Memory-Module/2037934408.html Ofther setting the klok, you dont need the arduino any more. Kind regarts, Bram |
| SilverSolder:
Perhaps go for a tiny micro... and find some more work for it to do, in order to justify its presence? - it could manage the whole test, control relays, etc., and make the whole calibration cycle more precisely repeatable. |
| Ian.M:
A 74HC4060 + a 32.768KHz watch crystal will give you a 2Hz output. Either redefine your spec for a 2Hz cal signal, or add another /2 stage (one D flipflop) after it. However, I'd go with the small MCU. Many have internal oscillators of +/-2% or better accuracy, which is likely to be superior to the 555. Use a PIC or AVR and after you've finalised your code, you can order them pre-programmed from MicrochipDirect*, so can treat them as a custom chip, no production line programming required, so no ICSP header either. For prototyping, patch on fly-wires to a loose ICSP header. Another option would be Analog Devices TimerBlox. A LTC6995-1 or -2 + three resistors to configure it to directly produce a 1Hz output. MicrochipDirect's price per programmed chip is quite reasonable, however the setup fee isn't worth it for very small runs, and as it applies to every code change, your code needs to be 100% complete, tested and frozen. |
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