Analog To Serial Converter

[Ilona Brownson]

Thereare several DAC architectures; the suitability of a DAC for a particular application is determined by figures of merit including: resolution, maximum sampling frequency and others. Digital-to-analog conversion can degrade a signal, so a DAC should be specified that has insignificant errors in terms of the application.

DACs are commonly used in music players to convert digital data streams into analog audio signals. They are also used in televisions and mobile phones to convert digital video data into analog video signals. These two applications use DACs at opposite ends of the frequency/resolution trade-off. The audio DAC is a low-frequency, high-resolution type while the video DAC is a high-frequency low- to medium-resolution type.

Discrete DACs (circuits constructed from multiple discrete electronic components instead of a packaged IC) would typically be extremely high-speed low-resolution power-hungry types, as used in military radar systems. Very high-speed test equipment, especially sampling oscilloscopes, may also use discrete DACs.

A DAC converts an abstract finite-precision number (usually a fixed-point binary number) into a physical quantity (e.g., a voltage or a pressure). In particular, DACs are often used to convert finite-precision time series data to a continually varying physical signal.

DACs and ADCs are part of an enabling technology that has contributed greatly to the digital revolution. To illustrate, consider a typical long-distance telephone call. The caller's voice is converted into an analog electrical signal by a microphone, then the analog signal is converted to a digital stream by an ADC. The digital stream is then divided into network packets where it may be sent along with other digital data, not necessarily audio. The packets are then received at the destination, but each packet may take a completely different route and may not even arrive at the destination in the correct time order. The digital voice data is then extracted from the packets and assembled into a digital data stream. A DAC converts this back into an analog electrical signal, which drives an audio amplifier, which in turn drives a speaker, which finally produces sound.

Most modern audio signals are stored in digital form (for example MP3s and CDs), and in order to be heard through speakers, they must be converted into an analog signal. DACs are therefore found in CD players, digital music players, and PC sound cards.

Specialist standalone DACs can also be found in high-end hi-fi systems. These normally take the digital output of a compatible CD player or dedicated transport (which is basically a CD player with no internal DAC) and convert the signal into an analog line-level output that can then be fed into an amplifier to drive speakers.

In voice over IP applications, the source must first be digitized for transmission, so it undergoes conversion via an ADC and is then reconstructed into analog using a DAC on the receiving party's end.

Video sampling tends to work on a completely different scale altogether thanks to the highly nonlinear response both of cathode ray tubes (for which the vast majority of digital video foundation work was targeted) and the human eye, using a "gamma curve" to provide an appearance of evenly distributed brightness steps across the display's full dynamic range - hence the need to use RAMDACs in computer video applications with deep enough color resolution to make engineering a hardcoded value into the DAC for each output level of each channel impractical (e.g. an Atari ST or Sega Genesis would require 24 such values; a 24-bit video card would need 768...). Given this inherent distortion, it is not unusual for a television or video projector to truthfully claim a linear contrast ratio (difference between darkest and brightest output levels) of 1000:1 or greater, equivalent to 10 bits of audio precision even though it may only accept signals with 8-bit precision and use an LCD panel that only represents 6 or 7 bits per channel.

Video signals from a digital source, such as a computer, must be converted to analog form if they are to be displayed on an analog monitor. As of 2007, analog inputs were more commonly used than digital, but this changed as flat panel displays with DVI and/or HDMI connections became more widespread.[citation needed] A video DAC is, however, incorporated in any digital video player with analog outputs. The DAC is usually integrated with some memory (RAM), which contains conversion tables for gamma correction, contrast and brightness, to make a device called a RAMDAC.

A one-bit mechanical actuator assumes two positions: one when on, another when off. The motion of several one-bit actuators can be combined and weighted with a whiffletree mechanism to produce finer steps. The IBM Selectric typewriter uses such a system.[1]

DACs are widely used in modern communication systems enabling the generation of digitally-defined transmission signals. High-speed DACs are used for mobile communications and ultra-high-speed DACs are employed in optical communications systems.

Other measurements, such as phase distortion and jitter, can also be very important for some applications, some of which (e.g. wireless data transmission, composite video) may even rely on accurate production of phase-adjusted signals.

Non-linear PCM encodings (A-law / μ-law, ADPCM, NICAM) attempt to improve their effective dynamic ranges by using logarithmic step sizes between the output signal strengths represented by each data bit. This trades greater quantization distortion of loud signals for better performance of quiet signals.

Talking to other IT guys about to analog converters, I hear a lot of horror stories. I looked online and they seem to be rather pricey too (as much as the fax machines). But at this point they seem like our only option.

The main problem with doing modem traffic (faxes are a type of modem) over VoIP is that modem traffic is FAR more sensitive to jitter than voice traffic. IP link quality levels that are perfectly acceptable for voice will be completely unsuitable for fax. Also, your line converters have to be able to recognize when a call is a fax call and switch the VoIP codec from the usual voice-quality one (which tolerates quite a bit of loss) to a much lower-loss fax-quality codec.

I have had better luck with faxes when I turn down the baud and turn off compression and error correction. Sometimes they have a overseas mode. Brother also has a cheap MFP that says it works well with IPT and it is not that bad.

The reason I do not use efax or any other similar solution is that they have a monthly cost. Our Financial controller does not like monthly payments (he pays everything late) that is why he is not approving the additional phone lines, but he was no problem with one time purchases.

Now what I have done to reduce the amount of fax lines. I bought a fax server. It is only used for incoming faxes. Anyone with a fax number now gets their faxes via email. Threw out all the basic fax machines. I have four mpfs that had faxing abilities. People want to fax hard copies they use these. Installed the fax print driver for the mfp on a server and shared it. Now they can fax from their pcs. Best of all, all four mpfs are on the same pots line

I am not sure I understand your question correctly. Are you looking to use the DAC8571 as an I2C master to read from your distance sensor, then output a voltage that can be connected to the analog input of your PLC?

If that is the case, then you cannot. The DAC8571 and sensor you have selected are both I2C slaves. This means that you will still need a master on the bus, such as a microcontroller. You could also see if you can get the PLC digital outputs to act as a master as well.

Hi!

Thank you for your reply!

Yes, that was my plan, but I understand now why it isn't executable. Thank you for your explanation!

The distance sensor has a digital output and uses an I2C communication protocol. Please correct me if I'm wrong, but I think it's impossible to directly connect the sensor output to an I/O PLC module.

Could you please recommend a microcontroller? Since I'm dealing with an industrial application, a robust solution is required.

Besides that, this project requires a large number of sensors (up to 30), so I need a relatively affordable way to connect all those sensors to the PLC (it seems expensive to use a microcontroller per sensor).

I don't understand how I can get the PLC digital output to act as a master. Could you please recommend any manuals/ documentation explaining how to accomplish that?

Thank you in advance!

You could consider an MSP430FR5969, which we have used in industrial applications in the past. The device could communicate to multiple I2C devices if they support multiple addresses. If you mean that you will have one sensor per PLC, then I think a lower cost solution should be considered. Otherwise, you could consider some other analog output sensors rather than the digital output.

I am not an expert on these PLC modules, but I see that some list that they have digital I/O, so it may be possible that they support some lower-level communication like I2C or SPI. Though I do not know for sure.

Thank you very much for your reply!

I pretend to use only one PLC. Sorry if I didnt clearly explain my idea.

I think S7 -1200 doesn't support I2C or SPI communication protocols. For what I've seen (in other forums), the most common solution is converting the digital output to an analog output, and then connect the analog signals to analog I/O modules. For instance, if the project requires 30 sensors, then 4 I/O modules (8 analog inputs per module) are needed.

I considered using other sensors that have analog outputs. However, I wasn't able to find any sensor that covers the whole required measuring range (if I decide to use sensors with analog outputs, I would need twice as many sensors, grouped in pairs. This solution would also require to double the number of I/O modules).

I'm still trying to find which solution suits my project the best (in terms of robustness and affordability).

If I go for the first option (sensors with I2C output), then I have the costs of the converters plus the microcontrollers (?). However, this solution requires fewer sensors as well as fewer I/O modules.

The second option doesn't have the conversion problem. However, I/O modules are quite expensive.

Could you please recommend any documentation on how to accomplish the digital to analog conversion? I don't understand how it works.

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