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DAC output not symmetric and noisy seen from scope


lukewang
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I have used clippers for a while and found many of factory default setting clippers have 2 issues for axis analog channel DACs:

(1)DAC output not symmetric, ie, one end could be +9V, and the other end could be -7.5V. we have to check and adjust for every newly arrived card

(2)I used scope to meansure DAC output and found its pretty noisy with lots of hi-frequency spikes

 

My quesions are

(1)if DAC output symmetry is a concern or we supose to tune it when it comes in

(2)if the noise on DAC port is acceptable and has any impact on axial control accuracy and stability?

 

My thanks in advance.

 

 

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lukewang,

 

The Clipper (Turbo PMAC2 Ethernet Lite) is using filtered PWM outputs and not true DACs. These analog outputs are generated by passing a PWM frequency through a filter. However, for this filter to work properly, you need to increase your PWM frequency to 30kHz. Considering that default factory setting has only 4.5kHz PWM frequency, the output will have lots of voltage ripple.

 

Please follow the instructions in the manual for Clipper, specifically settings mentioned on page 20 of manual under title "Filtered DAC Output Configuration".

 

Regards,

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Thanks Sina. It really helps.

 

Could you pls help me further on the following DAC issues:

For these DAC ports, we found on the same clipper card the output voltages are different from port to port, say, DAC1 max output can be 7.8V, and DAC2 may be 8.6V, with the default settings. and positive and negative outputs are not symmetic too. Any particular reasons behind? are we suppose to check and adjust each DAC port on each clipper?

 

Thanks.

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lukewang,

 

What is the PWM frequency setting in your Clipper? Please post the values for the following parameters:

 

I7000 // Servo IC 0 PWM Frequency Control

I7001 // Servo IC 0 Phase Clock Divider

I7002 // Servo IC 0 Servo Clock Divider

I10 // Servo Interrupt Time

 

Regards,

 

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Hi Sina & Brian,

 

Here are parameter settings on my clipper:

I7000=1001

I7001=5

I7002=3

I10=3421867

Ixx29=0

 

We tried to use Ixx29 to solve DAC zero drifting issue, is there better way to tune DAC zero output?

 

Thanks for your help.

 

Cheers,

 

Luke

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lukewang,

 

Please make sure that Ixx69=1024 on all motors. Then command #nOx where n is the motor number and x is a value between -100 to 100 and see if your output changes linearly.

If you can post the results.

 

We expect the following measurements:

 

#1o-100 >>>> output: -10V

#1o-60 >>>> output: -6V

#1o-20 >>>> output: -2V

#1o0 >>>> output: 0V

#1o20 >>>> output: 2V

#1o60 >>>> output: 6V

#1o100 >>>> output: 10V

 

Regards,

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Hi Sina,

 

Attached are the DAC test results and my clipper settings. Let me know your analysis.

 

Thanks.

 

Luke

 

Luke,

 

I checked the data that you sent and it seems correct. You have a Maximum Variance of 0.004948438 and Maximum Linearity Error of 2.90% which is expected for a filtered PWM output.

 

I would suggest that you increase your supply voltage from +/-12 to +/-15 and repeat your tests.

 

Regards,

Clipper DAC Data.xls

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