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| Amplitude modulation of AD9834 DDS - failed, but why? |
| (1/6) > >> |
| Yansi:
Hello, I've been experimenting with amplitude modulation of AD9834, but I have rather failed but don't know why. Based on this application note for a similar DDS: https://www.analog.com/media/en/technical-documentation/application-notes/AN-423.pdf and based on the datasheet of AD9834 directly, I have concluded the FSADJ pin leads to some kind of a current mirror, which sets the DAC FS current, but 18 times smaller. (the DAC full scale current is 18*1.15V/Rset) So what I've done was enhanced the circuit from that AN-423 with some precision. However the result was quite not expected. The DDS can not be fully modulated and it does weird sh!t. Below is the schematic of my experiment, below that a DSO capture of the voltage when the required modulation depth is small ) - the voltage on FSADJ does not change much (which is expected), the source voltage copies the input one well. When I try to apply more modulation, the FSADJ pin starts to go to GND, then to normal, then to AVDD (+5V) - what the hell is going on there? In such case, the output is not much modulated and modulation envelope is fairly distorted. I can get a maximum of about 20% modulation depth. I even should not be exceeding the maximum FSADJ current. Max DAC current is 4mA, which is 4/18 = 220uA FSADJ current. Scope shows about 400mV peak at R7, equaling 180uA peak current (or 3.24mA respectively). (FSADJ pin yellow, source of the 2N7002 in blue). Tnx for any hits with this. |
| MarkF:
I used this App Note to drive the FSADJ from 0V to 1.2V to control the amplitude of my AD9834. I'm just driving REFOUT with a DAC and a resistor voltage divider. The only difference is I only have a capacitor connected to REFOUT. I would disconnect your op-amp from REFOUT and see what you get. |
| Yansi:
You wanted to probably say you are driving FSADJ with a DAC, not REFOUT. REFOUT can not be driven as far as I can tell, but can be legitimately used as a reference voltage source, which is exactly what I used it for. Why should I disconnect my circuit from REFOUT? I am just using it as a source of precise voltage, sinking about 21uA from the pin. I think that is well within its capabilities. The reference pin is rock solid during all test, providing correct voltage of 1.2V. The mosfet modulator from the appnote is unusable with its ill-defined behavior (Vgsth spread, temp. sensitivity, etc). Hence I have tried improving it. Your circuit probably uses this trick: https://www.analog.com/en/design-center/reference-designs/hardware-reference-design/circuits-from-the-lab/cn0156.html#rd-description Both circuits work with the same idea - modulating current from the FSADJ pin. FSADJ should sit slightly lower than the reference voltage (guessing a current mirror must lie somewhere inside the chip). It should work the same way, whether I use voltage controlled current sink, or change voltage on the other end of resistor connected to the FSADJ pin. (In the latter case there will likely be high non-linearity when the amplitude approches zero magnitude). So why it ain't working in my case? |
| iMo:
Try to put 7002's drain at fixed 1.2V and measure its Ids current vs control voltage. |
| Yansi:
Yes, I am going to test that, but from the measured waveforms, it is kind of obvious, the problem is not in the voltage controlled sink. But yes, will verify that anyway. |
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