Interfacing Op-amp to DAC

Thread Starter

Cosmin Traicu

Joined Jan 6, 2017
12
Hello, i was trying to interface a DAC (LTC8043) with a op-amp (MCP6021) using the schema from attach. The problem is that Vout is always 0 volts. But if i use only the DAC (Op-amp disconnected), the voltage at Rf pin (= Vout) isn't 0.

Can anyone help my with this?DAC+Op-Amp.PNG
 

OBW0549

Joined Mar 2, 2015
3,566
Hello, i was trying to interface a DAC (LTC8043) with a op-amp (MCP6021) using the schema from attach. The problem is that Vout is always 0 volts. But if i use only the DAC (Op-amp disconnected), the voltage at Rf pin (= Vout) isn't 0.
That circuit produces an output of opposite polarity from Vref. With +2 volts on Vref, Vout would range from 0V to -2V, but since the op amp doesn't have a negative supply, it can't produce a negative output; so it just sits there doing nothing.

If you want a negative output voltage from this circuit, the op amp needs both + and - supplies, not + and ground.

If you want a positive output voltage, Vref must be negative rather than positive.
 

Thread Starter

Cosmin Traicu

Joined Jan 6, 2017
12
That circuit produces an output of opposite polarity from Vref. With +2 volts on Vref, Vout would range from 0V to -2V, but since the op amp doesn't have a negative supply, it can't produce a negative output; so it just sits there doing nothing.

If you want a negative output voltage from this circuit, the op amp needs both + and - supplies, not + and ground.

If you want a positive output voltage, Vref must be negative rather than positive.
My whole circuit is powered at 5V and voltage Vref is supplied by a voltage regulator, meaning that I can not set a negative voltage at the Vref pin. Is there another solution?
 

OBW0549

Joined Mar 2, 2015
3,566
My whole circuit is powered at 5V and voltage Vref is supplied by a voltage regulator, meaning that I can not set a negative voltage at the Vref pin. Is there another solution?
Then the design you showed will not work. Your best course of action, in my opinion, would be to use a voltage-output DAC such as an LTC1659.
 

crutschow

Joined Mar 14, 2008
34,283
Page 7 of this data sheet for an equivalent device shows how to use it in a voltage mode with the Vref used as the output and the reference voltage applied to Iout.
You then can use a non-inverting op amp circuit at the output as a buffer.
That will give you a positive output with only a positive voltage supply.
 

Thread Starter

Cosmin Traicu

Joined Jan 6, 2017
12
Page 7 of this data sheet for an equivalent device shows how to use it in a voltage mode with the Vref used as the output and the reference voltage applied to Iout.
You then can use a non-inverting op amp circuit at the output as a buffer.
That will give you a positive output with only a positive voltage supply.
Using this configuration would i get the same slew rate? Thanks you.
 
Top