DAC7760: DAC recommendation for wireless MCUs

Hello David,

DAC7760 is an application specific device targeted toward industrial 4-20mA loops and voltage analog output modules. It sounds to me like you are looking for a more general purpose DAC.

To my knowledge we do not have a device that could create the steps you require independently of the MCU. I think this will need to be done in firmware.

Do you have an idea of the resolution and accuracy your application requires? Also, what is the maximum output voltage required?

Thanks,
Garrett

Garrett Satterfield said:

Do you have an idea of the resolution and accuracy your application requires? Also, what is the maximum output voltage required?

Hello, Garrett.

- bit resolution: 10-bit or above. (12-bit is preferred)

- want to change the output in 10μs or faster; I expect SPI should be used. CC3220SF supports up to 20MHz clock speed max for SPI

As you see this picture, where the DAC's output was amplified using an Op Amp, the pulse width is 100μs but I need to change it quicker.

I need to create a 10μs width signal using TI's DAC.

- I need 4 DAC channels

- output voltage range: 0 ~ 3.333 V OR -1.66 ~ 1.66 V (curious whether there is a DAC that gives negative voltage so that I don't need to use a high pass filter)

Since I'm using CC3220SF-launchxl, I wish to 3.3V logic.

The power consumption doesn't matter for me. I'm gonna connect a power supply.

If the package is easy for populating on a breadboard, that would be great.

Thanks for your help. Hope to hear your recommendation, please.

Thanks, Garrett.

Garrett Satterfield said:

5us settling time

Q0. Is settling time equal to Sample/update rate (MSPS)?

Garrett Satterfield said:

DAC60504 - 12-bit, 4ch, 5us settling time, QFN

I think this will be perfect, except for the packaging.

Q1. Can DAC60004IPWR (14-TSSOP) be an alternative? I cannot find the difference between two regardless of the packaging.

Garrett Satterfield said:

DAC104S085 - 10-bit, 4ch, 6us settling time, VSSOP/WSON

DAC60504 - 12-bit, 4ch, 5us settling time, QFN

DAC61408 - 12-bit, 8ch, 12us settling time, QFN, high voltage bipolar output ranges

I thought I could do some simulation using TI TINA 9.

Garrett Satterfield said:

I have linked below a reference design and cookbook circuit guide that shows how to design a circuit to convert the unipolar DAC output to bipolar voltage output. These will be helpful if you want to use one of the first two options. Let me know if you have follow up questions.

http://www.ti.com/lit/an/slaa869/slaa869.pdf

Thanks for providing me this. However, it is not possible to simulate the aforementioned items.

Q2. Is there a way to stimulate the schematic from the cookbook?

My plan was to add OPA462 (±90 V) or OPA552PA (±30 V) instead of OPA188.

For instance, I need the output range of -27 ~ 27V by combining the DAC and OPA552PA.

Q3. How should I set the resistors to achieve this?

Q4. Also, I should I set the reference input? Do I have to connect an external power supply for that?

 

Q5. I haven't understood the power connection from the cookbook;

Since CC3220 use 3.3V, I thought I need to connect 3.3V to the VDD pin on DAC60004.

What should I connect on VDD; 5 or 3.3V?

 

Q6. Power-on-reset (POR) configuration: Connecting the POR pin to GND powers up all four DACs to zero scale.

Connecting this pin to VDD powers up all four DACs to midscale.

When applying SLAA869, where should I connect POR?

Q7. Or should I add a high pass filter and an Op Amp to amplify the output on each DAC outputs?

I'm curious what's the advantage of the SLAA869 compared to this method.

Hello David,

Find my responses to your questions below.

Q0. Is settling time equal to Sample/update rate (MSPS)?
A: No, the settling time specified is analog. The overall update rate will be the combination of the digital write time (which is dependent on SCLK) and the settling time of the DAC/Buffer output. Keep in mind the analog settling time changes based on load condition and step size. The datasheet specified it under very specific conditions.

Q1. Can DAC60004IPWR (14-TSSOP) be an alternative? I cannot find the difference between two regardless of the packaging.
A: Yes, the specs for DAC60004 are somewhat different and this device does not include an internal reference.

Q2. Is there a way to stimulate the schematic from the cookbook?
A: Yes, see the attached TINA files. They use an ideal voltage source for simulation purposes.

Q3. How should I set the resistors to achieve this?
A: Based on equations 2 and 3. If your reference is 2.5V select the ratio 27V/2.5V for RFB/RG1. Then use equation 3 to find RG2. It can be helpful to use series resistors if you need to achieve very close to the exact ratios. Alternately you can set the span wider (-28V, 28V) and calibrate.

Q4. Also, I should I set the reference input? Do I have to connect an external power supply for that?
A: Yes, for DAC60004 you will need an external reference voltage.

Q5. What should I connect on VDD; 5 or 3.3V?
A: Either. It depends on the value of the reference you choose. See the recommended operating conditions in the datasheet.

Q6. When applying SLAA869, where should I connect POR?
A: Most likely to VDD so that the startup voltage will be mid-scale which will yield ~0V output. But you could also connect it to GND and have negative full-scale output on start up.

Q7. Or should I add a high pass filter and an Op Amp to amplify the output on each DAC outputs?
A: I do not understand what you are describing. In the SLAA869 circuit you are basically doing this but offsetting to allow for bipolar output.

Thanks,
Garrett

Thanks, Garrett.

Garrett Satterfield said:

see the attached TINA files.

I cannot see your attached file. Can you re-upload them, please??

For this upload, can I request a schematic that has both DAC60504BRTET as the DAC and OPA552 as the Op Amp, please?

I want all DAC outputs to be bipolar. However, there is only one reference pin. So I wish to see your schematic and solution, please.

Garrett Satterfield said:

Then use equation 3 to find RG2. It can be helpful to use series resistors if you need to achieve very close to the exact ratios. Alternately you can set the span wider (-28V, 28V) and calibrate.

Curious whether this is possible with the TINA file.

Also, does TI has an SPI example code for configuring the DAC60504BRTET?

Garrett Satterfield said:

In the SLAA869 circuit you are basically doing this but offsetting to allow for bipolar output

Regardless of the application note, I mean, we can combine the high pass filter to make a bipolar output, right?

If I add a high pass filter to the DAC output and amplify it with an Op Amp, I think it is equivalent.

Compare to this way, what are the advantages of using SLAA869?

Hello David,

I was referring to the files linked at the end of the cookbook document. They provide some basic simulations but use an ideal voltage source instead of a DAC model. You can modify based on the op amp you are using.

http://www.ti.com/lit/zip/slac785

Q: I want all DAC outputs to be bipolar. However, there is only one reference pin. So I wish to see your schematic and solution, please.

A: It would be the same circuit with an op amp for each channel of the DAC. The reference voltage would be used for each to set the negative full scale.

Q: Also, does TI has an SPI example code for configuring the DAC60504BRTET?

A: We do not typically provide sample code as it can vary depending on the MCU you are using. It is fairly straight forward as long as you configure the MCU SPI hardware/software correctly.

Q: Compare to this way, what are the advantages of using SLAA869?

A: The advantage of the circuit in the cookbook is that it can create a DC bipolar output. You are correct you can use the DC blocking capacitor as you show, but you will not be able to control the amplitude of the waveform without changing the op amp gain. With the DAC circuit you could control the amplitude/waveshape via software. This may not be needed in your application, in which case you could use the MCU digital outputs with the circuit you show above.

Thanks,

Garrett

Hi, Garrett.

Garrett Satterfield said:

You can modify based on the op amp you are using.

Thanks! Mind if you can take a look before closing this thread? I also want to ask the pin functions as well.

I'm planning to use DAC60504BRTET from Digikey.

Garrett Satterfield said:

The reference voltage would be used for each to set the negative full scale.

Hope I understood your words for this.

Unipolar_BipolarOPA552.TSC

Q0. For the 2.5V reference, connect them like the above?

I noticed this instruction, so I added 180nF. Hope I did it in a proper way.

I didn't notice the DAC output range was 0~2.5V.

Q1. In order to get this output range, I have to connect 5V to VDD pin? Does this change regarding the GAIN pin?

Also, if I supply 3.3V to VIO, then I can use SPI in 3.3V level, right?

GAIN / RSTSEL / REFDIV / LDAC pins are quite confusing to me.

Q2. I want the DAC's output voltage to be 0 ~ 2.5V (gain = 1). Should I tie GAIN to GND?

Since both GAIN pin and GAIN register exists, this is making me confusing.

Q3. If I toggle (VIO 3.3V to GND), will the DAC be reset? "When connected to VIO all four DACs reset to midscale"

I'm not sure about the term midscale; what is this? Meaning running normally?

Q4. I want the DAC's reference voltage to be 2.5V. Should I tie REFDIV to GND?

Or do I just need to write 0 on REF-PWDWN bit?

I'm glad to see a good simulation result.