MAX038 Question

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#1
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Hi,

I just started thinking about building a function generator with MAX038.
The Output Frequency is calculated as followd: F0 = IIN(uA) / Cosc (pF).
IIn should range from 2uA to 750uA. That's not the problem.
But to get a Frequency Range from say: 1Hz to 20MHz the Capacitor as to change. So my question is: Is it possible to switch the Capacitor at the COSC Pin with an anlog switch or multiplexer (like MAX310)? Isn't the Ron of the switch a problem? How could i manage to control the full frequency range digital? Is there anything like a "digital adjustable capacitor" like there are digital potentiometers?

I don't understand how Maxim did a Frequency Range from 1kHz to 16MHz in 1kHz steps with a single 35pF Capacitor?

MAX038 Datasheet: http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1257
Application Note: http://www.maxim-ic.com/appnotes.cfm/appnote_number/650

Hava a look at my web page -> http://www.tobiscorner.at.tf

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The PSOC micro's from Cypress have something like a digital capacitor inside.

Open the pod bay doors, HAL.

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From my own experience with MAX038, specially if you use samples from them, is that you would need relays capables to handle low current (special contacts) and have the tracks from the 038 to the cap as short as possible.

And use a capacitor for decade, otherwise you would have a different frequency from the calculated since the f-iic curve has a big tolerance.

Guillem.

Guillem.
"Common sense is the least common of the senses" Anonymous.

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Hi Floe

Will you publish your project when done ?
I would be verry interested in a function generator with the MAX038 chip.

Cheers
Rubi

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Rubi wrote:

Will you publish your project when done ?

Sure, all my finished projects have been published on http://www.tobiscorner.at.tf and here at the Academy section.

I think i will try my first multiplexer methode. I can't think of another solution.
If i can't get it working i maybe switch over to an Analog Devices Chip. The AD9833 is a programable Waveform Generator (build with an DDS) from 0 to 12.5 MHz...

Hava a look at my web page -> http://www.tobiscorner.at.tf

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A good way to select between passive components is to tie all the leads together on one side. The connect each other lead on the cap or resistor to a port pin. Set the DDR of the entire port to all inputs. Then set the one port pin for the component that you want to select to output a logic zero. That component will be grounded. The other components are connected to a tri-stated port pin.

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Hmm, yes that should work. That's great, so i can combine the caps in every pattern (parallel) i want. I'll give it a try. Sounds like a great solution! Thank you

Hava a look at my web page -> http://www.tobiscorner.at.tf

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Simonetta: Can i use a PCF8574 I2C I/O Expander to save AVR port PINS. In the datasheet i can not see if input pins are tri-stated...

Hava a look at my web page -> http://www.tobiscorner.at.tf

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@theFloe

As i see it
You need to make sure that you can tristate each individual pin.

/Bingo

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I can set each individual pin as input or as output. What i don't know is, if the pin is tri-stated if configured as input...

Hava a look at my web page -> http://www.tobiscorner.at.tf

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theFloe wrote:
Hi,

I don't understand how Maxim did a Frequency Range from 1kHz to 16MHz in 1kHz steps with a single 35pF Capacitor?

MAX038 Datasheet: http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1257
Application Note: http://www.maxim-ic.com/appnotes.cfm/appnote_number/650

Hi,

IC2 and IC4 together makes a constant current source for coarse frequency adjustment. Then,
there is IC1, it's programmable divider and contains phase comparator. IC3 is some sort of
filter, and makes frequency fine adjustment. The circuit is basically a phase locked loop (PLL) .
You can replace the freq. switches to AVR's outputs.

And sorry for my bad english ;-)

- Tommi

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With the 35pF capacitor, the frequency range will be approx 60kHz-20MHz,
so there is no way to get down to 1kHz with the 35pF C_osc. Actually, the MAXIM
schematic shows a variable 35pF C_osc, so presumably they mean you need to
increase capacitance to (at least) 2000pF to get down to 1kHz. My experience
with switched in capacitors using HF relays was quite satisfactory (dead-bug layout).
The sine wave quality suffers at frequencies > 10MHz. I was able to get a range
of 50Hz - 12MHz by switching in two capacitors (3300pF and 0.1uF) in parallel to a
33pF fixed C_osc using a pair of SPDT HF relays. If you really do not want to
use relays or switches you may use two or more MAX038 chips (together with
a video mux if necessary). I have no experience with the idea of tri-stating and
grounding an AVR (or PIC) pin as suggested by Simonetta.

There are many possibilities for using digital frequency synthesis with the MAX038,
other than the approach shown in the MAX038 datasheet (which uses a Motorola
PLL chip). E.g. use a 74HC4046 with a divide-by-N external counter (which can
possibly be a 20MHz AVR). Or implement a software PLL using an AVR.
You can even generate the reference clock for a PLL using software DDS with
an AVR with very high frequency resolution. Etc.

For complete digital control (if you willing to go through the trouble) you can use
three DACs controlling I_in, F_adj, and D_adj pins. An AVR (or PIC) uC can
continuously measure the frequency and duty cycle output by the MAX038 and
make adjustments as needed. You will still need to switch in capacitors for
lower frequency ranges.

If you don't need the triangle and sawtooth of the MAX038, it is proabably simpler
to use an AD98xx DDS chip (but it is a tiny SMD).

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Thanks, i just started looking at the AD9833. The DDS can output sin, traingle and square waves at frequencies from 0Hz to 12,5MHz in 28bit resolution. I think thats enough...

Hava a look at my web page -> http://www.tobiscorner.at.tf

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Dean 94TT
"Life is just one damn thing after another" Elbert Hubbard (1856 - 1915)

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Hi

I got the AD9833 , but the package is really, verry verry small.
After routing an adapter in Eagle, I am ready to work with this little bugger.
Are there any special considerations for the Layout, when the Generator should work within the Range of up to 12,5 Mhz so that the IC can produce a "clean" signal ?

Cheers
Rubi

Attachment(s): 

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Where did you get the AD9833 ? I asked AD for samples but will need to find some locally if the project I'm looking at works ok.

Ralph Hilton

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I ordered them as Samples from AD...

Hava a look at my web page -> http://www.tobiscorner.at.tf

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I ordered them from an Ebay seller :-(
Although they have not been expensive :-)
Even though they had to travell from Florida to Vienna 8)

Cheers
Rubi

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I'm impressed! I got 2 samples from AD which arrived in 8 days. That is pretty good given that I'm 90Km from a fair size city and up in the snow and ice of the Austrian Alps.
I wrote some code after reading the datasheet and it worked fine the first go even with the poor thing suspended in midair on a bit of stripboard. I fed its clock from XTAL2 of an ATmega8 at 16MHz and it was fine.

Ralph Hilton