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

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

just a quicky to let you know that I just bought 7x AD7822s`, all the others were SMD devices (Useless to me).

they only cost £2 each from a fella in Israel.

these should be sufficient for now as back-ups are always handy, and may even lead to some single use stand-alone devices later.

Again, thank you for your help

edited to update: Scratch that, turns out the AD7822 were SMD devices also, so I canceled that and bought 4X ADC1175s` instead, I figure if I`m forced to go the SMD route I may as well have something I want and hopefully get a breakout bourd for it and solder it up without wrecking too many of them in the process.

just a quicky to let you know that I just bought 7x AD7822s`, all the others were SMD devices (Useless to me).

they only cost £2 each from a fella in Israel.

these should be sufficient for now as back-ups are always handy, and may even lead to some single use stand-alone devices later.

Again, thank you for your help

edited to update: Scratch that, turns out the AD7822 were SMD devices also, so I canceled that and bought 4X ADC1175s` instead, I figure if I`m forced to go the SMD route I may as well have something I want and hopefully get a breakout bourd for it and solder it up without wrecking too many of them in the process.

You are most welcome. I am glad that I could help.

Nick Gray