Hello everybody,

I have to make a 5-bit ADC and DAC using only op-amps, encoders and flip-flops. I've been doing some research and there is a lot of designs, specially for ADCs. I would like your advise on the most efficient design I could build using the elements I mentioned before.

PD: Mainly, this ADC/DAC is to transform a signal from a heart beat rate sensor which I have to make also using only op-amps. However, my teacher told me he's gonna test my converters with other signals as well.

Thanks in advance.

 

There are several ways to approach the problem. Are you familiar with any of them? Is a comparator included in the op-amp class?

 

Yes, specially with the flash ADC, but since it is a 5-bit converter I am gonna need >32 op-amps, so I'd like to avoid this...

And yes I can use everything which is made with op amps, that includes comparators

 

Yes, specially with the flash ADC, but since it is a 5-bit converter I am gonna need >32 op-amps, so I'd like to avoid this...

And yes I can use everything which is made with op amps, that includes comparators

You need to google the SAR (Successive Approximation Register) technique. It will use far fewer op amps.

https://en.wikipedia.org/wiki/Resistor_ladder
https://en.wikipedia.org/wiki/Successive_approximation_ADC

Actually this technique gives you a twofer since it has both a D/A and an A/D converter.

 

Another method to consider, which uses just one op amp and a comparator plus some timing circuitry, is the Integrating ADC.
It's slower than the SAR method but should still be sufficient in your application for heart rate sensor.
It allows relatively high accuracy with a simple circuit, and has an advantage that the integration filters any high frequency noise in the signal.
It's the method used in a lot of digital multimeters.

 

Another method to consider, which uses just one op amp and a comparator plus some timing circuitry, is the Integrating ADC.
It's slower than the SAR method but should still be sufficient in your application for heart rate sensor.
It allows relatively high accuracy with a simple circuit, and has an advantage that the integration filters any high frequency noise in the signal.
It's the method used in a lot of digital multimeters.

This is also an excellent suggestion.

https://www.maximintegrated.com/en/app-notes/index.mvp/id/1041

 

Another advantage of the Integrating ADC, as mentioned in PB's reference, is that the integration time can be selected to notch out 50 or 60 Hz interference, which can be a problem in heart sensor circuits.

 

Hello, I have completed a working 5-bit SAR ADC-DAC. However, I don´t know what the frequency ratio between the SAR's clock and the S/H's(sample and hold) should be. So if I have a (A)Hz signal what should the S/H and SAR clocks frequency,respectively, be? in order to get the best quality in the output signal.

Hope you can help me out. Thanks in advance

 

The ratio between the S/H clock and the SAR clock is determined by how many SAR clocks it takes to perform one conversion.

 

So if I have a (A)Hz signal what should the S/H and SAR clocks frequency,respectively, be? in order to get the best quality in the output signal.

Wouldn't the sampling frequency be determined by the characteristics of the heart rate sensor signal (electrocardiogram?) and the intended use for the digitized signal? The heart sensor signal may have a period of only ½ second yet there will be much higher frequency components in that signal; however, at some point the signal strength of the higher frequency components will become lost in the conversion noise of the 5-bit converter. Were these issues supposed to be part of the design task, or did the assignment specify a sampling frequency?

 

Since you didn't say that you could use any type of code, SAR is out (unless you can use encoders and flip-flops to create the hard-code necessary). The simplist is a 5 bit flash converter, but it will use 32 opamps.

Either get busy creating the hard-code out of the components you have, or bite the bullet and get those 32 opamps out of the parts bin.