Hello,

I need a voltage reference to set the offset of an instrumentation op amp (TI's INA 122).

The datasheet shows a schematics to do it (see p. 7, top right).

However I wonder if I I really need the REF200 reference or if I could replace all the parts on the right of the OPA336 with a simple variable resistor divider (with a big resistance such as 1Mohm to limit consumption) inserted between the power rails. Or maybe just replace the REF200 with Zeners?

Thank you in advance.

 

A voltage divider is ratiometric with respect to the power supply that feeds it. If it changes x%, so does the output of the divider.

 

A potential divider or zener would not be as accurate or stable as a proper reference source.

 

OK. Would you advice any alternative (better, cheaper ?) component/range of components to achieve this instead of theREF200?

 

Are you running it with a single supply or with + and - supplies?

I use the LM385-2.5 fairly often.

What is your drift spec?

 

The datasheet said most applications don't require external offset adjustment. Do you really need it? If you do, how accurate/stable do you require it to be?

If I was using a $6 opamp, I'd be inclined to use the recommended offset adjustment if I needed it...

 

LM4040 is specified down to 0.1%. What is the output of the inamp used for? What is your error budget for the DC offset, in either millivolts or %?

ak

 

Without a requirement for the stability of the offset in your application, it's not possible to give you a good answer.

 

Repeat, repeat, repeat...what is your error budget?
Nobody can tell you which parts will meet your needs unless you tell us your needs.

 

That circuit on page 7 of the data sheet is only required if you want to trim out the offset for best accuracy. It is a precision circuit and only worth using if you need it. Using a non-precision circuit may make the offset worse so you might as well use the simple circuit on page 1, Vref connected to 0V. Whatever you put on Vref pin will be the reference for the output pin Vo.

 

The purpose of this circuit is to digitize sensors signals for a gesture control interface in the context of performing arts. I shall use a dual supply (+/- 5V).

I need the offset to be stable on the long run because the rehearsing sessions might last several hours but very fast variations will probably be filtered by a low pass below 500 kHz or even less. The global range of the digitized signals will be calibrated in a microcontroller each time the device is switched on (3.3V ADC with 12 bits).

 

If you want a precision offset adjustment you have to use a precision reference; no point in using a resistor or a zener which will make things worse.

Also, the INA 122 has a bandwidth that is dependent on gain, see page 2 of the data sheet; 500kHz is out of range at any gain.

 

500kHz is out of range at any gain.

As written, it could be lower - the figure was just a rough indication.

So I understand I need a precision reference. Again, which component / components range would you generally recommend for that purpose ?

 

The purpose of this circuit is to digitize sensors signals for a gesture control interface in the context of performing arts. I shall use a dual supply (+/- 5V).

I need the offset to be stable on the long run because the rehearsing sessions might last several hours but very fast variations will probably be filtered by a low pass below 500 kHz or even less. The global range of the digitized signals will be calibrated in a microcontroller each time the device is switched on (3.3V ADC with 12 bits).

I assume you need the offset to be only in one direction (polarity). If so then I would go with an inexpensive voltage reference, such as the TL431 (us$0.39 @ DigiKey), to minimize any possible problem with voltage drift.
The TL-431 is a shunt regulator which acts like a very accurate zener diode.

Edit: Just noticed that the LM4040 is a similar device at about the same price, so either should be fine.

 

I have just used the LM4040 to remove offset in a new design. Don't have any figures yet for actual performance but I note that AK mentions it above. So 2 votes for LM4040.

 

Thank you!