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

I do not know just what you mean by "high quality" because this means different things to different people. However, here are some 16-bit DACs with I2C interface:

AK4440 octal DAC from AKM Semiconductor; AD5629R and AD5669R octal DACs and AD5543 quad DACs from Analog Devices; LTC2605 and LTC2657 octal DACs from Linear Technology and the DAC8574 quad DAC from Texax Instruments. The AKM device is 24 bits, while the others are 16-bits.

- Nicholas "Nick" Gray

Thanks Nick,

Although this part does have a high output impedance as you point out, the input capacitance of the ADC is only 15 pF (typ) which gives a time constant of only 9 nS for 600 ohms; I'm not sure how this would translate to the 11 mS settling I'm seeing.  Looking at this another way, say I need a 4 volt step on the output.  According to  the data sheet, the LMP2011 can source about 9mA at 4V.  Thus,  i=C*dv/dt=>

     dt=15pF*4/.009=6.7nS.  That's a million times less than 11mS.

And it doesn't look like a 'normal' settling time issue where you would see an overshoot and some damped ringing-the output stays 'stuck' at some constant level for 11mS, and then rises to the 'correct' value.  It looks more like what you'd see when an opamp recovers from saturation (the output is well below saturation-about 300mV with a 5V supply).

Ironically, National says in their data sheet: "The LMP201x is a great choce for an amplifer stage immediately before the input fo an ADC."

I will definitely look into alternate opamps.  Also, I think I can live with setting my sample rate lower so that I can accomodate the long settling time.  At this point I'm more concerned about WHY this didn't work as expected; in my experience, when you don't understand a problem but think you've found a fix, you haven't really.

Barry

Dear Sir,

Can you explain to me what is the function of this device  DG201 CMOS QUAD SPST ANALOG SWITCH? I had search using google or yahoo  search machine but still cant find any site explaining this device except the DATASHEET. The DATASHEET did not explain in details the theory behind it. So  if you know what is it please tell me or paste the link for me here.Thanks..

Hi Nick

Really appreciated that u replied my message.I'm not sure whether we are able to wait for the launching of this chip but that will be wonderful if this chip is made available in the market soon. Our's is an asia based company mainly focusing on bio equip such as dialyser but the targeted annual volume is still in consideration. i would like to once again thank you on this reply and hope to receive more information on the new chip.

Best Regards

Alvin

Hi Steven,

A tip: just type it into Google, I did that and found: http://www.intersil.com/products/deviceinfo.asp?pn=dg201

SPST = Single Pole, Single Throw (1 in, 1 out).

Its a simple voltage controlled switch.

Regards,

John.

Hi, Stephen -

I am not familiar with Proteus, but I do suggest that you contact Lamcenter Electronics, the supplier of this software, and ask them. They do have a user's forum that may be able to help. See them at http://support.labcenter.co.uk/forums/viewforum.php?f=9&sid=9c3487a09b6bb86720bd111886391d57.

I have attached hera an Excel spreadsheet which simylates the AD633, based upon information from the AD633 data sheet. This spreadsheet generates random inputs each time you change anything. The easiest way to do this is to select an unused cell and press the DELETE key. I hope it helps.

- Nicholas "Nick" Gray

Hi, Stephen -

The function of an analog switch is basically the same as a mechanical relay, except that the analog switch is a solid state device using MOSFETs. The analog switch generally does have a higher input to output capacitance than does a mechanical relay, which allows high frequency signals to pass when the switchis "open". A good tutorial on the analog switch can be found at www.analog.com/static/imported-files/tutorials/MT-088.pdf.

- Nicholas "Nick" Gray

Hello, Alvin -

If you will please send me the name of the company needing the load cell Signal Conditioning Chip and your email address, I will be sure that you have the information on this device as soon as it is available. When you have an estimate of the expected annual usage, please let me know that as well.

I had said that I knew of no sensor signal conditioning chip available now, but I found that that a company in Germany named ZMD is making some, although they are not at all as good as the solution I told you about that will be available in a few months. You can go to the ZMD web site at http://www.zmdi.com/.

Best Regards,

Nicholas "Nick" Gray

Barry -

I phoned a friend of my in the amplifiers group of National Semiconductor and he told me that the LMP2011 has a lot of capacitance associated with it (it is a chopper-stabilized amplifier) and the overload recovery time is typically 50 ms. See data sheet pages 3 and 4 for overload recovery time specifications. The key to getting what you need is to insure that the part never goes into overload. That is, keep the device operating within its linear range. This implies ensuring that the input does not momentarily make a jump so large that the amplifier has difficulty following it, and being sure that the output never saturates. I suggest you may need an amplifier that is NOT chopper stabilized, but such an amplifier would have a larger input offset voltage.

- Nick Gray

Barry -

I phoned a friend of my in the amplifiers group of National Semiconductor and he told me that the LMP2011 has a lot of capacitance associated with it (it is a chopper-stabilized amplifier) and the overload recovery time is typically 50 ms. See data sheet pages 3 and 4 for overload recovery time specifications. The key to getting what you need is to insure that the part never goes into overload. That is, keep the device operating within its linear range. This implies ensuring that the input does not momentarily make a jump so large that the amplifier has difficulty following it, and being sure that the output never saturates. I suggest you may need an amplifier that is NOT chopper stabilized, but such an amplifier would have a larger input offset voltage.

- Nick Gray