Have a question about ADCs or DACs? Ask our Expert, Nick Gray

Hi Mukund,

I am not sure just what you want to do. I know you want to convert the radio waves coming from your mobile phone, but I do not know what you want to do with the energy once you "convert" it. If you want to decode the information coming from it, you really need the equivalent of a mobile system base station, which is a LOT of circuitry. The information transmitted in addition to the voice or text information includes a lot of things, including information needed to know when to change receive and transmit frequencies when the mobile phone leave one cell area and enters another.

If you mean convert it to electrical energy for general use, like replacing a battery or something, than that is not practical because it the energe generated is so small that it is almost completely useless.

If you mean something besides one of these, please explain just what you want to do and ask again.

Nick Gray

Hi Nick

I would like your help with the selection of a suitable ADC IC for a sensor product I am working on.

I  need to accurately read (16 bit), the analog output of 2, 5 volt hall  effect sensors, the output of which swings from 0.5 to 4.5 volts. The  output of the sensors changes very slowly over time so I don't need  anything fast. I can allow up to about 2 seconds for the conversion. My  device wakes up every hour, reads the sensors, stores the result and  then goes back to sleep.

I'm  using a PIC18F46J11 as my controller which runs at 3.3 volts but I also  have a 5 volt supply on the board to power the 2 hall sensors. I'm  using SPI for interfacing to all the other components so the ADC also  needs to be SPI.

Ideally  the ADC would be able to interface directly to the pic with 3.3v SPI  lines, but the analog side to be able to cope with the 5 volt input of  the sensors.

I know there are various types of ADC but I'm not sure which would be most suitable for this application.

Many thanks, Ben Harker

For example ADS1146 and similar:

http://www.ti.com/product/ads1146

Hi, Ben,
A couple of ADCs that should do well in your application with Hall Effect sensors are the ADS1146 and the ADS1255, both from Texas Instruments. I am sure that other suppliers probably have suitable devices as well. However, I think that it would be much less expensive and use less board area if you combined the PIC and ADC in a single package by using one of the CY8C34 family of Programmable System-on-Chip (PSoC) devices from Cypress Semiconductor. Digi-Key shows pricing at less than half of the price they show for the ADS1146 or the ADS1255, in addition to the board area and inventory cost savings. The PSoC devices can operate from supply voltages of 1.7V to 5.5V and have separate analog and digital supplies. The analog supply must be equat to or greater than the digital supply, so operating the digital at 3.3V and the analog at 5V is absolutely no problem.

The PSoC can be shut down to consume almost no power when not needed. It has an 8051 core, up to 64k of Flash memory, up to 8k of RAM, have SPI, I2C, USB and CAN interfaces, have a Delta Sigma ADC that can be programmed for resolutions of 12 to 20 bits, has timers, counters, PWM, four DACs (voltage or current output), four op-amps, four voltage comparators, sample-and-hold, temperature sensor, programmable analog blocks that can be used to build any number of analog functions, LCD direct drive circuit, CapSense inputs, up/down mixer and just about anything else you need for mixed signal systems. The PSoC Creator software allows you to configure your PSoC just the way you want to without worrying about how to make your custom internal connections. Create your schematic in the PSoC Creator software, then tell the software to configure your device. This is really a fast, easy way to go.

          Vout = Vterm + [ (Vref – Vterm) * Load R ] /(Rdac + Load R)

where Vterm is as described above, Vref is the DAC reference voltage, Load R is the resistance of your load and Rdac is a nominal 10k, with minimum and maximum of 8k and 12k, as described in the data sheet page 16. The above formula is valid for any value of Vterm between 0V (ground) and the DAC supply voltage.

The probable reasons you are seeing no output include the following:

  Your load resistance is very low, Vterm is 0V and you are measuring the voltage output to ground or output to Vterm
  Vterm is the same as Vref and you are measuring a voltage across the load or the current through the load

Please know that the output voltage of this type of DAC should always be maintained at 0V to get accurate current output, which is why all of the applications in the data sheet show the use of op-amps as current-to-voltage converters.

I suggest that you look at the AD5410 to source a current. It has output current ranges of 0 to 20mA, 0 to 24mA and 4 to 20mA.

If you want to sink a current, I suggest you try the DAC-08. It can sink up to 4mA as long as its output is always at least 5V above its negative supply.

Hi,

I`m after an 8 bit A to D converter that will run up to a couple of MHz (but nothing past 4Mhz as it would be a waste).

I`v already got the Audio A to Ds working well for my computer, but whilst experimenting with them to get it running smoothly, I couldn`t help but wonder what would happen if I attatched my Long-Wire antenna to the input, and surprisingly there Was radio of sorts present.

needless to say that prompted a few ideas

my D to A is near infinate speed as it`s a homemade R2R, so that side is fine, but now I want something that`s a Single channel, 8 bit, really fast conversion time, A to D.

the voltage rails aren`t a problem as I`v been forced to use a PC PSU for 5V current reasons, so have all the available +/- 12 and 5v stuff ready and waiting.

I have a ZN425E-8 here, but feel certain there are likely to be Other chips out there that will out perform this, so to save me reading Tons of data sheets, does any particular chip spring to mind?

Cheers

The reason that you picked up radio signals when connecting a long wire antenna to the input is because the ADC had an input bandwidth that could pick up those signals. The sampling action of the ADC then aliased the input signal, converting the RF signal to the audio band as the signal was digitized. You can “tune” in different stations by adjusting the ADC clock frequency. If the ADC input bandwidth is too high, you may sometimes pick up two adjacent radio stations at the same time, although one or both of them may be a little distorted.

Most ADCs today have a serial output. This allows them to be in a much smaller package with fewer pins, reducing their cost. Because you have used the ZN425E8, I am assuming that you want the ease of use of a parallel output ADC and will not consider serial output devices.

There seems to be a gap in ADC speeds between 1 Msps (megasamples per second) and 10 Msps, with not many products available in that range. However, all ADCs can operate a little faster than their specified maximum speed, but their performance may suffer if you try to operate them too much faster than their rated speed. On the other hand, high speed converters may not work well at very low speeds. The general solution to this type of problem is to use a high speed converter and just lot look at all the samples if you do not want data a high rates. For example, If you want data at 2 Msps and can not find a suitable converter at that speed, get a converter specified at 15 or 20 Msps, operate it at 16 Msps and look at every 8th output. This gives you a used data rate of 16 Msps / 8 = 2 Msps. Don’t consider the use of a super fast ADC as a waste. The prices of these things are about the same as slower devices. Here are some suggested ADCs The first is from Analog Devices, the others are from TI / National Semiconductor.

AD7822, 2 Msps
ADC1173CIMTC/NOPBADC1173CIMTC/NOPB, 15 Msps
ADC1175, 20 Msps
ADC08351, 42 Msps
TLV5535, 35 Msps
TLC5540, 40 Msps
ADS830EADS830E, 60 Msps

Nick Gray

That`s fantastic, Thanks!

currently I`m using a Z80 at 8.192 MHz, but hope to upgrade to a 10 MHz version soon, so any even multiple of that would be great, and who knows, since the entire CPU card is replacable, using a Fast A to D would make it somewhat "Future proof".

Cheers

I am happy that I could help. My best to you.

Nick Gray