AD9850 module setting the amplitude of the voltage and offset - Page 1
Forum section: electronics / projects

author: kulamario
author_rank: Newbie
timestamp: September 18, 2015, 03:58:09 pm
content: WelcomeI'm being built function generator module already finished controlled by the SPI bought on ebay, the code to control've already got ready for testing sine and square shows great, but do not know how to solve the problem of amplitude adjustment and offset, he wanted me to have an adjustable amplitude say from 0V pk-pk to 10V pk-pk, because at this moment the module sinusie out of 1V pk-pk, and square 5Vpk-pk.I do not know, I have no idea how to make this adjustment, can someone with his suggested something to me, it may take a opamp?Maybe some of you I've already done a generator with adjustable and can share schemePlease hellp me

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 18, 2015, 07:28:43 pm
content: as per our PM, i've put a "quite sloppy" method to do that, in that thread. there a another lot better way to do it, such as using "voltage gain control" opamp which i havent touch...

author: Kleinstein
author_rank: Super Contributor
timestamp: September 18, 2015, 07:30:45 pm
content: There are 3 options to do the amplitude adjustment:1) Use the ref. voltage input of the AD9850 to adjust the amplitude - at least the fine part, like 1:4 range. The coarse range switching sould better be done with switchable amplifiers or attenuators behind the filter.2) Use a varable amplifier to do the whole adjustment. There are some that can be adjusted via a voltage, somewhat similar to an analog mulipier.3) Use a manual pot to set the amplidude - though at the higher frequencies this might no tbe the best way. The potentiometer schould be rather low impedance (e.g. 200 Oms) to work at high frequency.Depending on how the amplitude is set, it might help to have means of measuring the amplitudeAdding an Offset ist much easier: just add a DC voltage , just before the output amplifier. An alternative might be shifting the DC voltage at the GND terminal of the output connector.

author: babysitter
author_rank: Frequent Contributor
timestamp: September 18, 2015, 07:37:38 pm
content: What range do you want?You can find cheap 75 ohm fixed and adjustable attenuators, around 0-20 dB attenuation, for cable tv. Might be the cheapest way to go.

author: kulamario
author_rank: Newbie
timestamp: September 19, 2015, 06:58:24 am
content: WelcomeI wanted to have a set from 0V to 10V sin and square

author: Rerouter
author_rank: Super Contributor
timestamp: September 20, 2015, 05:51:16 am
content: Here is the output stage of my own variation on it, Literally i built the unit it is part of to be a brick shit-house that almost nothing could knock out,The top pot controls amplitude, i made it for +-25V, but you can play with R19 to get where you want, as that controls the offset, It was at least intended for me to be either DC, negative or positive, so that bit can probably be improved,

author: void_error
author_rank: Frequent Contributor
timestamp: September 20, 2015, 07:08:01 am
content: I think your op amp might be a bit slow, I mean way too slow, unless you don't need to go above a few tens of kilohertz...

author: Kleinstein
author_rank: Super Contributor
timestamp: September 20, 2015, 08:02:51 am
content: For a resonably fast output one essentially needs to have a defined output impedance. This is typically 50 Ohms. So you have an Amplifier with a low output impedance and than something like 47-50 Ohms as a series resistor.The resistor usually also ensures short circuit protection for the output.Because of the output resistance, the output voltage depends on the load - under nominal, matched load the voltage allready halves compared to the open circuit case. With AC voltage the value can be given as effective value or as peak-peak reading. Both ways are common but the peak-Peak value is about 2,8 times larger.So there is not just one output voltage.One big question is still if the amplidute should be set by hand, like with a mechanical potentiometer or under controll of the µC, which is the more difficult way.The easy way is having a mechanical potentiometer to set the amplitude and than a sufficiently fast and powerful amplifier followed by the output resistor.

author: Rerouter
author_rank: Super Contributor
timestamp: September 20, 2015, 09:29:05 am
content: Sorry, to clarify, i was using this for low speeds, but at a wide voltage range with an easy to adjust current limit, For Automotive testing purposes.The circuit can still be applied to faster op amps,. and he can strip away stupp like the optomosfet and back to back diodes, the ptc and tvs diode, etc, like i said this is what i have done and it works beautifully, have put the outputs across mains and it just tripped the circuit breaker, hooked it up to a 24V battery system with no hassles, etc,