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Old 28th October 2011, 06:52 PM   #1
Aiace is offline Aiace  Italy
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Default PCM1794 strange current output

The data sheet of PC1794A says that the current of each 4 ot the output is 6,2mA. I've just measured the current output and I've found 1 chipe that has all 8mA and another one has 2 output with 7,45ma and 2 output of 8,1mA
Strange I thought. But the better is yet to come....
When I've tried to join two output pins into a differential fashion the current didn't double ! Infact I've measured about 11mA

So I'm wondering and asking to you why the corrent haven't doubled when I've joined two pins ?

thank you in advance
Aiace
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Old 28th October 2011, 10:20 PM   #2
Aiace is offline Aiace  Italy
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hemm....PCM1794 and not PC1794.
sorry
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Old 28th October 2011, 10:50 PM   #3
jcx is offline jcx  United States
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did you measure with the speced DAC ouput, "burden" V - too much V drop in the current measurement may cause error
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Old 29th October 2011, 12:18 AM   #4
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How did you measure that current? What signal was at the input of the DAC, what was the resistance of your mA probe...
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Old 29th October 2011, 12:33 AM   #5
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The outputs of the PCM1794 and 1792 are held at a constant -6.2mA. This is what they mean in the datasheet when they list the BPZ current (bipolar zero current) as -6.2mA.

The outputs then swing +- 3.9mA around that value to give you your output swing. That is the output will swing between -2.3mA to -10.1mA.

The outputs of the DAC are also expecting to see zero load impedance.

If you use the analogue filter circuit as proposed in the datasheet, the first two opamps perform current to voltage conversion at a rate as determined by the value of the feedback resistor.

That is, if you're using a 750 ohm resistor 750x-6.2mA will give you a constant DC offset after the first opamps of -4.65 volts for both the +ve and -ve output halves of the DAC.

The next opamp is configured as a difference amplifier, as the DC offset is common to both the +ve and -ve outputs it is cancelled out leaving you with negligible DC offset. The analogue signal on the other hand is equal but opposite for the +ve and -ve halves so they sum constructively leaving you with the analogue signal.

The resistor connected to pin 20 at a value of 10k to ground, is necessary for biasing the internal output circuitry. If this value is different, IE larger, the output current will drop. I would not recommend making it any smaller as the DAC could explode.
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Old 29th October 2011, 05:35 PM   #6
Aiace is offline Aiace  Italy
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I meadured it at zero BPZ usign a digital multimeter stright to the PCM1794A pins and nothing else is connected to them. The manual of multimeter writes that it has a voltage drop of 5mV/1mA, so it should have 5 ohm of resistance. I suppuse this is low enaught.

Strange.....any idea?
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Old 29th October 2011, 07:09 PM   #7
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I think you need to connect the pins on the outputs so that they are allowed to sink current properly. If you measure the idle current consumption of the DACs analogue 5v supply I think you will find that it differs greatly depending on if the outputs are connected up properly or not.
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Old 29th October 2011, 08:29 PM   #8
kevinkr is offline kevinkr  United States
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I just popped in and fixed the thread title.

5th element is correct, and I suspect that the resistance of your meter is also high enough to cause some measurement errors - also I would not expect very good accuracy from generic DMM at relatively low current levels - do you know the specified accuracy of your meter?
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Old 29th October 2011, 10:33 PM   #9
Aiace is offline Aiace  Italy
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The accuracy is 1.2% of rdg + 3 digits

PS: thank you to fixed the title
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Old 29th October 2011, 11:30 PM   #10
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Do you have a 20mA scale? When was the last calibration done?

Anyway, the datasheet specify +/-2% error for DC center current - that is not related at all with the DAC accuracy in AC. Adding the 5-10% from your meter you can get impressive numbers. Try measuring those mA in both directions, see if you get equal numbers (one positive, one negative). That will tell you how accurate the mesurements are on your meter.

Last edited by SoNic_real_one; 29th October 2011 at 11:34 PM.
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