Few questions about the DCA0800 (Digtal/Analog Converter)

[dbk229]

Hello,

I have to design a circuit with a DCA0800 component.

I read the datasheet and some application note but I have a few questions about how it works.

Here is a link to the datasheet :
https://www.electro-tech-online.com/custompdfs/2007/10/DAC0800.pdf

The schematic of the component :



My questions are :

- How do I chose Vref+ and Vref- ? What will be the difference between a 5V or a 15V differential voltage ?

-How do I chose V+ and V- ? Because on the output I only use the Iout currents (who are set with Iref).
So what will be the difference between a +/-5V or a +/-15V supply ?

- Do I have the choice for the Iref value ? What will be the difference if I chose Iref=2mA or Iref=1mA (speed, consumption) ?

- On the output I'll have to transform the current into a voltage. In the datasheet they show different possibilities : with an AOP or directly with 2 resistor.
What are the advantages of using an AOP ?



The DCA0800 seems to be very customizable but I have some difficulties on choosing the settings

Thank you for your answers

 

[ericgibbs]

My questions are :

- How do I chose Vref+ and Vref- ? What will be the difference between a 5V or a 15V differential voltage ?

-How do I chose V+ and V- ? Because on the output I only use the Iout currents (who are set with Iref).
So what will be the difference between a +/-5V or a +/-15V supply ?

- Do I have the choice for the Iref value ? What will be the difference if I chose Iref=2mA or Iref=1mA (speed, consumption) ?

- On the output I'll have to transform the current into a voltage. In the datasheet they show different possibilities : with an AOP or directly with 2 resistor.
What are the advantages of using an AOP ?

hi,

If you chose a +/-5V supply, the Rref resistor would be 5V/0.002= 2.5K ie: 2mA Iref current.

So select the Rref so that a Iref is set as 2mA...

The +/-V supplies will determine the output swing range of the ADC.

I would always use a Buffer OPA on the outputs, so that the circuit the ADC is driving will not load the ADC output.

I would consider using +/-5V supplies.

OK.?

 

[dbk229]

Ok for the use of an OPA on the output and the supply voltage (I forgot to tell you I will use a +/- 15V supply, but it doesn't change anything).

For Iref apparently I don't have the choice, it's 2mA and nothing else ?

For Vref I can chose whatever I want, the important is that I get Iref=2mA ?
What's going to happen if I chose Vref+ = 4V and Vref- =-1V. Does my circuit will still run ?

 

[dbk229]

Does anybody knows if the Vref voltages are important while I have Iref=2mA

 

[ericgibbs]

Does anybody knows if the Vref voltages are important while I have Iref=2mA

hi,
Do you want the output of the DAC to be a bipolar signal?

If yes, what output voltage range do you require.?

What are the voltage levels of the parallel digital inputs.?

 

[dbk229]

Yes I would like a +/- 1V on the output.

I will apply a 5V signal on the digital input (I don't see what it has to do with the Vref voltage ?)

 

[ericgibbs]

Yes I would like a +/- 1V on the output.

I will apply a 5V signal on the digital input (I don't see what it has to do with the Vref voltage ?)

Its because it sets the centre point/level of the output swing.

Why dont you just use +4V for the Vref , use a resistor in series which will enable a 2mA current, its explained on 4,5 and 7 of the datasheet.

On your Binary input is 00000000 = to -1V and 11111111 = to +1V out

or is the Binary weighted as Figure 8.???

 

[dbk229]

On your Binary input is 00000000 = to -1V and 11111111 = to +1V out

Yes that's it.

Yes I could use a +4V Vref, but I still don't understand what Vref- is used for.



On the schematic they show us a voltage comparator and I don't understand its role.
If I take a Vref- level too high I'll have voltage level too low on the output of my comparator ? All my bipolar transistors won't be able to draw enough current ?

 

[ericgibbs]

hi,
For TTL logic inputs, connect VLC to 0v.

Set the Iref input to 1mA, with a +4v source use a 4K resistor to the Vref input pin and connect -Vref pin to 0v via a 4K resistor .

Look at page 4 of the datasheet at the Ifs/Iref graph.

On the I out pins connect a 1K resistor to 0v on each pin, this should give a +/-1V swing on the output voltage for a binary input code of 00h to FFh.

I would connect an OPA on the I outputs in order to buffer the DAC.

The Vref is misnamed IMO, its really a stable external voltage source which is used to drive the Iref current into the Vref pin. A series resistor from this external source sets the input Iref current and the OUTPUT currents..

Any external stable +V voltage source can be used to drive the +Iref input providing the current is limited to the specification in the d/sheet.

The lower the power supply to the DAC the lower the possible output current that be given at the Iouts, See the graph Page 4.

Yes I could use a +4V Vref, but I still don't understand what Vref- is used for.

The +Vref and -Vref are the inputs to a 'long tail pair' transistor amplifier.

This type of transistor amplifier can accept bipolar inputs, also it can be used with a signal input on only one input, the other input is then connected to 0V or common via a resistor.
So in the DAC circuit +Vref is the pin with the signal input ie: 1mA ,,, the other pin -Vref is connected to 0V.

In order to give the optimum operation of the long tailed pair amplifier, the unused pin ie: -Vref is connected to 0V thru the same value resistor as the +Vref