When building AVR DDS2 signal generator there were lots of discussions about signal conditioning in analog part of device. First argument was that LM358 wasn’t the best choice for this purpose. Another one pointed to sine wave that weren’t smooth enough.
As you can see there are some dents on it. Other waveforms also are distorted especially when higher voltages are selected. This definitely asks for better analog part. Some people suggested to replace LM358 with OPA2134, but it seems to be quite expensive choice. In my opinion low noise general purpose op-amp can be great too. I’m gonna give a try to Texas Instruments TL074 low noise op-amp. It is low power, high slew rate (13V/us) IC – almost five times faster than LM358 and for same reasonable price.
If we look at DDS2 signal generator offset and gain control circuit we can see that there is a problem in it. We regulate offset before amplitude control – normally offset should be regulated after amplifying signal. And third problem is that offset can be regulated in small interval – in 5V range while we could do this in -12V … +12V range. Lets redesign analog part so it would give better result.
Zeroing offset voltage
First of all offset control we can slightly modify first stage so that offset voltage of signal would be set to 0. For instance if we are generating sine-wave on DAC we get maximum 2.5V amplitude signal with 2.5V DC offset. Instead of using POT1 we can calculate resistor divider so that on output we get 0V offset. We assume that all signals coming out of microcontroller have offset 2.5V (when VCC=5V), then we can model following circuit:
Calculating is easy:
We know that source voltage is Vs=2.5V. Output voltage is Vo=0V. We need gain 1 (-1 for inverting amp). So at inverting op – amp input we get voltage:
From from operational amp analysis we know that V- = V+ and currents to both inputs are equal to zero (I- = I+ = 0A). Then We must get 1.25V at V3. so we need to select voltage divider to scale voltage from 5V to 1.25. To do so we chose one fixed resistor value R3 to be 100kOhms and then we calculate R4:
We can get 33.33kΩ by connecting two resistors in series: 33kΩ + 330Ω that will give resulting voltage close enough to 0V.
Optional filter stage
In the next stage of analog part we are going to add a filter. Since DDS signal generator is capable of outputting various types of signals we don’t want it to be hardwired. For instance if we generate sine wave the we could use filter to make it more smooth while on square wave it would have negative effect by rounding the shape. For this we can add a bypass switch that would allow to insert or bypass filter on demand. What filter are we going to use for this purpose. Since we have plenty of op-amps on TL074 we can make an active filter.
I find Butterworth (Sallen – Key) low pass filter quite handy since it gives flattest response in band. Lets select parameters for this filter. DDS generator in our case isn’t going to exceed 100kHz frequency. Also we want attenuation in this up to roll – off frequency close 0dB. So lets increase roll-off frequency up to 200kHz. This value we are going to use while calculating.
Resistor and capacitor values of filter can be determined with these simple rules:
R1 = R2, C1 = 2·C2
If we select R2 = R1 = 33k (have plenty of these)
then we can calculate C2 as follows:
so standard value of C1 = 33pF.
You can check calculations on line here.
Signal amplitude gain control
After we have adjusted signal offset voltage filtered it (or not) the next part is to adjust amplitude. We need to adjust signal amplitude from 0 to 12V. For this purpose we are gonna use inverting amplifier with potentiometer adjustable gain. Wee need to calculate proper resistor values to get nice gain control over all potentiometer turn. Say we are going to use 47k potentiometer. Lets calculate input resistor value.
Another known condition is that input voltage from previous stage is 2.5V. Say we want to get 12V amplitude on output we need gain: 12/2.5 = 4.8. If we turn potentiometer to the max left we get:
R1=47k/4.8=9.79k~10k. In order to get 0V amplitude we just need to turn potentiometer to the right so gain ration gets close to 0.
Signal offset regulation
And last stage of analog part is signal offset regulation. We want to regulate offset in range of -12V to 12V. The easiest way to do so is to add offset voltage to signal voltage.
Since we already have two inverting cascades we don’t want the last one to be inverted that would lead to inverted signal on output. So we are going to implement non inverting summing:
Lets see how to calculate resistor values. R6 and R7 we select to be 100k resistors as they aren’t critical while in recommended range 1k to 1M. More interesting part is gain. Lets see how non inverting summing amplifier output voltage look like:
What we see here is a voltages added and multiplied by gain. Since our R7 and R8 are equal to 100k we get that only half of these voltages are added. So we need to adjust amplifier gain to 2 in order to operate with full values. So we need:
after solving we get that both resistors have to be equal. In order to keep resistor values less scattered we also chose those to be 100k.
Putting it all together
Now we have all blocks ready and can connect them in to single circuit:
Here we have connected all four modeled parts: offset adjust, low pass filter, amplitude control, ofset control. TL074 chip comes with four operational amplifiers built in so we will get away with single chip and this functionality. If you are interested in simulating here is LTspiece simulation file. In simulator TL074 were replaced with similar TL1359 op amp, so actual results shouldn’t differ much.
And corresponding signals on various nodes:
As you can see on first stage we have a sine signal with small distortion added and its offset is 2.5V. After offset adjust signal still is distorted but it crosses 0V now. After filtering we see no more distortion – only smooth sine. Then next stage adjusts signal voltage gain and on output we get signal with selected offset somewhere at -5V. Results looks promising so next step will be to put everything in to working project. Comments and suggestions are welcome.