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Can ADR280 Voltage Ref be the AD8226 ref terminal?

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tony_lth

tony_lth

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Hi,
I want to set a voltage to the AD8226 REF terminal.
From AD8226 datasheet, it says, "For the best performance, source impedance to the REF terminal should be kept below 2 Ω."
ADR280 is a voltage ref at my hand, can its output directly link to the ref pin of AD8226?
Best,
Tony Liu

- - - Updated - - -

I want to set 1.2V as ref to AD8226.
 

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  • ADR280.pdf
    842.4 KB · Views: 89
  • AD8226ARMZ_datasheet.pdf
    415.6 KB · Views: 74

Hi,

a complete circuit of input signal connection (with all the voltages and impedances) to the target (ADC?) can help to give better assistance.

***
What we can see:
The Reference_IC can supply up to 100uA.The amplifier has an input resistance of 50kOhms.
If we just take the DC current limit, then the voltage across the 50KOhms resistor must never exceed 5V. (We don´t know without additional informatins on your circuit)

If we look for AC, then I doubt your reference can handle high frequency. (We don´t know about any frequency in your circuit)

--> Without additional informatins ... I recommend to use a buffer.

Klaus
 
OK. I will give you circuits SCH tomorrow.
The AD8226 uses max gain, i.e. 51 ohm as gain resistor.
And the signal is almost as DC, less than ten Hz.
AD8226 uses +3.3V and -3.3V as power supply.
Under these conditions, do you think ADR280 can directly link to AD8226? I only need AD8226 ref be between 1.2V and 1.65V.
Best,
 

Hi,

I wonder why you want to add 1.2V ... 1.65V of offset to the amplified output signal... But schematic will show.
Usually REF is connected to GND.

Datasheet says:
The transfer function of the AD8226 is
VOUT = G(VIN+ − VIN−) + VREF
Click to expand...


****
Under these conditions, do you think ADR280 can directly link to AD8226?
Click to expand...
I assume: Yes
But i can not recommend it now.. without schematic.

Klaus
 
The reference IC isn't designed to sink current. Depending on the AD8226 common mode input voltage, you may want to add a load resistance to the reference output.
 
Hi, KlausST,
Here is the SCH for your comment.

- - - Updated - - -

And AGND and GND is linked together somewhere else.

- - - Updated - - -

And @FvM,
IN- and IN+ are AC-coupled without common mode input voltage, and even with it, it should be out of control.
So I only want to set a stable Ref of AD8226.

Yes, I want to set Ref to 1.65V or 1.2V as DC bias of output signal.

- - - Updated - - -

I found another LDO, ADP121-ACBZ165R7. It can set Vout to 1.65V. So I want to directly link ADP121 output to the REF pin of AD8226.
Do you think it is OK as REF terminal of AD8226?

- - - Updated - - -

I attached ADP121 datasheet here.
 

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  • 捕获1.JPG
    捕获1.JPG
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  • ADP121.pdf
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Hi,

The schematic doesn't show node (opamp datasheet) voltage, nor does it show where OUT goes to.
It doesn't show why you want to add the voltage.
It doesn't show used input voltage (range)

***
An LDO is no REF. If you need a REF, then use a REF. If you need a supply then use a LDO.
The LDO has no sink current. ( like FvM says about ADR280)

First check if you really need the .
Then either use a new REF with enough drive capability, or use AD2 80 and a buffer.
Don't use an LDO.

Klaus
 
IN- and IN+ are AC-coupled without common mode input voltage, and even with it, it should be out of control.
Click to expand...
1. An AC coupled instrumentation amp needs common mode bias resistors
2. You should add a load resistor for the reference to guarantee a positive output current, e.g. 24k for 50 µA.
 

Hi, KlausST, I need DC bias for ADC sample, because ADC range is 0-3.3V, but my signal maybe positive or negative.
So I need it.

Hi, FvM, Actually IN- and IN+ have 20K to GND, which is your "common bias resistors"?
From AD8226 datasheet, ref pin only needs 7uA current. And ADR280 can output 16uA, so I add a 130K resistor so that its passes 9uA current and the other 7uA current for AD8226. DO you think this SCH OK?

- - - Updated - - -

Or could you recommend one voltage reference IC?
 

Attachments

  • 捕获3.JPG
    捕获3.JPG
    118 KB · Views: 86

Hi,

I need DC bias for ADC sample, because ADC range is 0-3.3V, but my signal maybe positive or negative.
Click to expand...
That's why I asked for.. I assumed something like that.

---> With this information I strongly recommend to you use the same REF as for the ADC. Use a simple 1:1 resistive voltage divider and a buffer.
This gives the best results.
Why:
* A fluctuation in the ADC Vref will result in unknown new offset and noise (AC and DC) in digital results
* A fluctuation in the opamp Vref will result in unknown new offset and noise (AC and DC) in digital results

* with the use of one common ref the offset change is minimised (about zero, because compensated). It is fixed and easy to compensate with software
* use two resistors of the same batch. This give the best long time stability and best temperature stability.
* It is a simple and cheap circuit.

Klaus
 
I found ADR127 is OK, which can supply mA current. Do you have any comment?

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Hi, KlausST,
Thanks for the comment. I think the fluctuation of ADR127 is in our tolerance range, and for simple, so I want to directly link ADR127 output to AD8226 ref pin. Of course, with FvM suggestion, I will add a 1.8K resistor on the output for 700uA current, to meet AD8226 7uA ref current. Do you have any more comment?
Many thanks.
 

Attachments

  • ADR121_125_127.pd.pdf
    597.8 KB · Views: 72

Hi,

My comment: please reread post 10.
An additional Ref will add new errors while it gives no improvement.

Klaus
 
And,
ADR1581 is cheaper, and I want to use it to replace ADR127.

- - - Updated - - -

Hi, KlausST,
Thanks. I know the error and it is in the control.

Except the noise increase, any other comments?
 

Attachments

  • ADR1581.pdf
    897.4 KB · Views: 72

Hi,

ADR1581 is cheaper
Click to expand...
Cheaper than what? A simple OPAMP?

***
ADR1581:
You can use it. It will work. With all it´s disadvantages as written before.

Klaus
 
Hi, KlausST,
Thanks,

- - - Updated - - -

I want to try.
 

KlausST said:
Hi,

What we can see:
The Reference_IC can supply up to 100uA.The amplifier has an input resistance of 50kOhms.
Click to expand...
Hi, KlausST,
I checked AD8226 Ref current, the datasheet says that its typical current is 7uA. So I wonder where you get the info that it needs 100uA? If it's 7uA, and because ADR280 can supply 16uA max, so I think it should be ok that ADR280 be the ref IC of AD8226.
Pls double confirm.
Thanks.
 

Hi,

I wrote:
The Reference_IC can supply up to 100uA.
Click to expand...
The reference_IC is the ADR280.
ADR280 datasheet doesn't tell much about source current.
But below figure 13 it says, that it can drive 100uA.

I never said that the AD8226 needs 100uA current.
I said:
The amplifier has an input resistance of 50kOhms.
Click to expand...
The amplifier is AD8226.
To know how much current this means, you need to know the voltage at the other side of the 5k resistor.
Then you can calculate the current with the help of ohm's law.

Klaus
 
HI, KlausST,
That is clear. Thanks.
Best,
Tony Liu
 

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