using pseudo bipolar ADC

bluerabbit97 wrote: > I've some questions with regards to pseudo bipolar ADC. >

It would be more helpful to name the chip and the task at hand. John Larkin gave a pretty clear explanation of how pseudo bipolar works. An opamp doing some gain and offset on the signal should get you squared away.

GG

Hi, Blue. Mr. Larkin explained it pretty clearly.

A "pseudo-bipolar" or differential input ADC means one which accepts two signals, and gives a digital output which is the difference between them. This is actually a two input ADC, with capability to digitally subtract. That's all.

Since your ADC is almost certainly single supply, each input can only extend from the power supply rail (or your voltage reference) to GND. Going below the negative of the single supply is against the rules, and will smoke the IC.

You can get an ADC reading from a +5V to -5V signal using a voltage divider, again as Mr. Larkin suggested (view in fixed font or M$ Notepad):

| | 1K 1% | ___ | >-|___|-o------>

| Sig. In | To ADC | (+5V/-5V) .-. (+5V/0V) | | | | 1K 1% | | | '-' | | | o | +5V | (created by AACircuit v1.28.6 beta 04/19/05

With the voltage divider, you don't need a differential input ADC at all. One input will do the job.

If you have further questions, please include more information about your application, and be sure to include which ADC you're using. It's easier to help if both sides know.

Good luck Chris

Thanks GG & Chris for replying. Sorry for not including the details. I've not work with ADC before so I'm confused.

I'm currently using ADS1217 ADC by TI and a gyro sensor. The gyro sensor outputs a maximum of -5v to +5v, through a single signal line, depending on the rotating direction.

I'm using the gyro in a slow angular rotation application so I expect a max output of around +/-2.5v.

I've read the ADC datasheet and it states that the "Analog Input Voltage" accepts voltage between AGND+/- and AVdd+/-.

Does that means I can't directly connect the gyro output to the ADC input?

Thanks Blue

Hi, Blue. First off, I guess I should say I've never used the delta-sigma ADC you've specified (although I've used others), and the comments below are based on perusal of the data sheet. Take with a grain of salt -- trust, but verify.

The ADS1217 is a single supply (analog) and single supply (digital) ADC. Assuming you're using a 5V supply for AVdd, the electrical characteristics on p.2 basically say you can't have any of the input voltages exceed AVdd or go below AGND. Actually, with the buffer ON, you can't go below 0.05V or above 3.5V. Of course, that means you can't directly tie your sensor output to the ADC input -- you'll smoke the IC.

Actually, unless you've got other really good reasons for using this chip, it doesn't seem to be the best choice for your application.

There are ways of making this work, though. The whole thing is really based on having 0V to 2.5V input signals, or less if you're using the programmable gain array. You might want to use the 2.5V reference voltage from the ADS1217 and an external single supply precision op amp to do some level shifting and get your +/-5V signal to do 1.25V

If you could take the time, you might want to describe more about your application -- i.e.:

• Are you going to be gathering data at 60Hz (or 50Hz, whatever your local line frequency is)?

• Do you REALLY need 22 bits (or 19 bits) of effective resolution?

• Is a 0.00000V output from your gyro significant?

ADCs are fun, but just having a high-resolution delta-sigma ADC by itself doesn't solve all your problems. In fact, it can frequently create more (voice of experience).

You also might want to try cross-posting at s.e.d -- for sure someone there has used the IC.

Good luck Chris

Hi Chris, I'm currently doing a school project and I need the gyro for vehicle navigation, in total there will be 3 gyro, one for each axis, therefore I need a high resolution ADC.

As for the sampling rate, it is about 5~10Hz sampling rate. Output voltage would be in mV significant, 0.000V significant.

might want to use the 2.5V reference voltage from the ADS1217 and an external single supply precision op amp to do some level shifting and get your +/-5V signal to do 1.25V +/-1.25V (ADC input range 0-2.5V), as mentioned in another post.

I've thought about it but I'm not sure if I got it correct, please correct me. Thanks.:

*For the non-inverting op-amp, the output op-amp voltage should follow the +ve gyro output voltage. It will clipped the

-ve gyro output voltage to 0V. This non-inverting op-amp output will then be fed into the Vin(+) of the differential ADC.

Actually I've tried out another ADC, AD7738. This ADC's input can accept +/-2.5V but I can't get any response from it after connecting up the recommended circuit provided in its datasheet. I'm using dsPIC30F6014 as the microcontroller and using SPI to communicate with it.

Thanks Blue

OK, Blue. This might be fun. A couple of questions first:

• College or trade school, and is this the senior project?

• How long do you have until it's due?

• Could you provide the make&model of the gyros?

Good luck Chris

Hi Chris,

I'm in university doing Final Year Project.

due?

This ADC is just a side track that is needed for the whole project. I wish to complete in a month's time.

gyros?

Currently I'm not given the make model of the gyro, just an insight that it will provide a max of +/-5v. I was told to get the ADC tested using a sine-wave generator.

Thanks Blue

Hi, Blue. Here's something that might get you started, if you have to use the single supply ADC.

Let's assume gyro zero is significant, but you have to get pretty much a full scale range from -5V to 5V, with inputs pretty much limited to

-2.5V to +2.5V. You can start with the idea above of using a voltage divider to level shift, and work from there.

Without further information, we'll have to assume your gyro output center is actuually at 0.000V, rather than (Vcc + Vee) / 2. Find this out -- it's critically important, and you'll have to change your plan if it isn't.

I'm going to also make the assumption that you've got other analog voltages available to power things, like, say, a +/-12V for the gyro.

Doing the problem this way, you'll have to start out with a good quad op amp with low voltage offset and drift (U1). Choice of op amp is dependent on your power supplies, and what you have available now.

You'll need a reliable +5V reference, and there are a number to choose from. But you've already got a 2.5V reference on chip, and you can use the op amp to make that 5V, like this (view in fixed font or M$ Notepad):

| | 2.5V |\\| | o------|+\\ 5.00V | | >--o---o | .--|-/ | | | |/| .-. | | | | 10K 0.1% | |1/4 U1 | | | | '-' | | | | '--------o | | | .-. | | |10K 0.1% | | | | '-' | | | === | GND | (created by AACircuit v1.28.6 beta 04/19/05

In this setup, the important thing is stability of the voltage reference, not precision.

Now, look for analog switch IC (U2) that will switch bipolar voltages with a logic level control signal from your uC. High OFF resistance is more important than low ON resistance. Choose the one that works for you based on availability. Remember that pinouts for these ICs are somewhat standardized, so you can always drop something in and order an upgrade.

You'll need time for the rest of the show -- try to get something up and running from a hardware standpoint as soon as possible, so you can start learning what else you need.

By obtaining these components, you can set up a unipolar signal input of 0 to +2.5V with center or zero at 1.25V, and typical signal riding from 0.625V to 1.875V and a uC-controlled autozero calibration like this:

| | .-----. | | | 20K 0.1% X 2 | Gyro |1/4U2| ___ ___ 5.Vref | o------------o o---o---|___|-o-|___|----o | -5V/5V | | | | | | | | | | '--o--' | | | | | | 1/4U1 | |\\ | | | | .-----| >O--' | | |\\| 20K 0.1% X 2 | | |/ | '---|+\\ ___ | | | | >---o-|___|-o----o | | | .---|-/ | | To ADC | | .-----. | | |/| | .-. | | | | | | | | | | | |1/4U2| | | | | | | | .----o o---' '----------o '-' | | | | | | | | | | | === | | === '--o--' GND | | GND | U1 = Quad low offset low drift op amp | CAL | | U2 = Quad analog switch | o---o-----------' | | (created by AACircuit v1.28.6 beta 04/19/05

This should give you a good start -- one of these for each of your 3 axes. Since your ADC is faster than your data requirements, you can occasionally switch CAL on, and do an autozero cycle to be able to cancel out offset and drift. One quad op amp IC, two quad analog switch ICs, a handful of precision resistors in easily obtained values, and you're done.

The devil, as always, is in the details on this. To get a realistic precision of 1 or 2mV, you'll have to be particularly careful with ground paths. Ask your adviser for other suggestions on this. Take the time to read Analog Devices AN-345, AN-404 and the venerable AN-202, "An IC Amplifier User's Guide to Decoupling, Grounding, and Making Things Go Right for a Change" By Paul Brokaw.

Good luck, and feel free to post again if you run into problems or if the assumptions about the gyro are incorrect. Hurry early.

Good luck Chris

Hi Chris,

Thanks for your advice. I hope to get it work soon. Will post here if I do.

Thanks Blue