View Full Version : fast dac for propeller?

07-23-2009, 03:33 AM
Can someone suggest a fast dac for the propeller?ˇ 8bits ok.

I'd like 100ns settling time and a DIP package.ˇ Preferably i2c.

I saw the Maxim max518 and it's perfect for breadboarding since it's i2c and 2 dacs in an 8pinˇdip but the settling time is 6us.ˇ Too slow for my application.ˇ

Iˇgenerate waveforms with 2us features.ˇ

Nick Mueller
07-23-2009, 03:42 AM
I2C and 100ns sounds tough! http://forums.parallax.com/images/smilies/smile.gif
I suggest looking at www.analog.com or www.ti.com.
Maxim are famous for announcing at least 10 times more than they can actually deliver or produce. .-/


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Bill Henning
07-23-2009, 04:05 AM
You must use a parallel dac - 2us features are impossible with i2c and almost impossible with SPI (possible with the 20Mbps output trick)

See http://www.national.com/mpf/DA/DAC0808.html 150ns settling time, 8 bit parallel dac

See www.analog.com/static/imported-files/data_sheets/DAC08.pdf for 85ns settling time 8 bit parallel dac

henry99 said...
Can someone suggest a fast dac for the propeller? 8bits ok.

I'd like 100ns settling time and a DIP package. Preferably i2c.

I saw the Maxim max518 and it's perfect for breadboarding since it's i2c and 2 dacs in an 8pin dip but the settling time is 6us. Too slow for my application.

I generate waveforms with 2us features.

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Post Edited (Bill Henning) : 7/22/2009 9:32:49 PM GMT

07-23-2009, 04:22 AM
I doubt if you will find a fast 8-bit DAC. Linear Technology makes very fast 12-bit DACs, you could simply use the lower 8-bits.


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Phil Pilgrim (PhiPi)
07-23-2009, 04:30 AM
I have to question the combination of 100ns settling time with an I2C interface. It will take at least 40us just to transmit a new data word to the DAC with I2C. So nothing that uses I2C is going to be any where near "fast".


07-23-2009, 05:32 AM
Take a look at the mcp4725 its SMT but sparkfun have a breakout http://www.sparkfun.com/commerce/product_info.php?products_id=8736.
It supports a 3.4Mhz I2C mode but you would have to write your own driver for that. I dont know of an existing object that will run I2C that fast, it still will not meet your settling time of 100ns - its settling time is 6us.

07-23-2009, 07:15 AM
Ok I think I2C probably won't work very well with the 100ns DAC.

What kind of performance do you think I could get using a resistor and cap as a low pass filter and using the propeller as 1 bit DAC?

My analog circuits is rusty but I remember the transfer function ofˇC/Rˇ+ response toˇtake the inverse laplace transform to calculate the resulting circuit.


1) Is there a simple simulation somewhere where I just put in valuesˇfor R/C and the freq of the PWM + duty cycle and see the output?

2) I'd like to get 4us at say 1V then ramp up to 2V over 10us then hold it for 4us then ramp back down to 1V over 10us (trap waveform).ˇ Do you think the RC circuit + PWM freq of 125MHz with variable duty cycle as shown in the examples will give me the performance I want?

3) How sensitive is the circuit to the values of R and C?ˇ Do I need super high tolerance parts to get good performance that follow the simulation details?

4) Do you think the 1 bit dac would work (maybe I need a higher order low pass?) or should I just stick with an 8 bit parallel dac with 100ns settling time?

Thanks for all the help!