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Again on NOS TDA1543 DAC active filter
Again on NOS TDA1543 DAC active filter
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Old 15th October 2018, 09:54 AM   #11
ygg-it is offline ygg-it  Italy
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Quote:
Originally Posted by DF96 View Post
If you are using an opamp to do the IV then this stage needs to implement some filtering because an opamp output cannot follow the transitions at the output of the DAC. It need not be the main filter, though.
Ok, will use a fast opamp unity gain stable IV converter supplied with +/-15 volt. As usual in all my project I will use AD826 (not AD828).

I need an advise: feedback resistor have to stay 1100 ohm (1.1kohm) and the cap across it 4.7 nF. This sounds perfect with another DAC. I won't change.

Can you please suggest me:

1) optimum supply voltage for TDA1543. I calculated 5.53 V or more (6V?)

With an assumed output current of 2.3mA, this results in a maximum voltage swing of 2.53 volt (1.1K * 2.3mA). The AC voltage swing has to stay within 1.8V and Vsupply-1.2V.
So 1.8V+2.53V+1.2V=5.53V min

2) optimum Rbias value connected as:
Ground -> Rbias ->TDA1543_vref -> AD826_non_inverting_input

I found 2.69K independently of voltage supply.

I need a biascurrent of 1.64 mA (1.8V/1100), so Iref needs to be 0.818mA, resulting in a Rref of 2.69Kohm = [2.2V/(1.64mA/2)].

Correct?

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Old 15th October 2018, 10:43 AM   #12
DF96 is offline DF96  England
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You mention using an opamp for IV, then a resistor. Which? If an opamp virtual ground then you don't have to worry about AC voltage swing on the DAC chip output because this will be approximately zero.

My calculation (done a few years ago) is that if you want 1.1k output resistor then you need a minimum supply rail of 7V.
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Old 15th October 2018, 11:30 AM   #13
ygg-it is offline ygg-it  Italy
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Quote:
Originally Posted by DF96 View Post
You mention using an opamp for IV, then a resistor. Which? If an opamp virtual ground then you don't have to worry about AC voltage swing on the DAC chip output because this will be approximately zero.

My calculation (done a few years ago) is that if you want 1.1k output resistor then you need a minimum supply rail of 7V.
I will use exacly the solution in the datasheet, thus an opamp with 1.1k of rfeedback (instead of 1.2k) and a cap of 4.7nF (instead of 3.3nF).

So which is the optimum supply and Rbias from ground to pin 7?
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Old 15th October 2018, 12:39 PM   #14
DF96 is offline DF96  England
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There is no optimum supply if you are using an opamp for IV.
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Old 17th October 2018, 09:03 PM   #15
ygg-it is offline ygg-it  Italy
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Quote:
Originally Posted by DF96 View Post
There is no optimum supply if you are using an opamp for IV.
Ok, thanks.

Can you please give me a simply rule of thumb to select Rbias from pin 7 to ground by using an opamp as IV converter with 1.1K feedback? Datasheet says nothing. Someone don't put at all. Someone suggested even 22k....

I think that it should increase Midpoint some way..

Is this correct?

Midpoint=2.2V+1.1K*1.15mA+1.1K*2.2V/Rbias*2 (??)

So if I want 4V midpoint should I need Rbias=7.5k?
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Old 18th October 2018, 02:27 PM   #16
DF96 is offline DF96  England
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Sorry, it is a long time since I studied the datasheet.
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