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I have an application where my Arduino is controlling a mcp4725 DAC for a design replacing a sensor on an old car. The DAC provides the analog output voltage, .25V to 4.9V to the computer. I recently measured the OEM device I'm looking at replacing and the sensor output had 40mA of current on the line.

Because the MCP4725 DAC only is good for 25mA I need to increase the output current of my DAC, MCP4725.

After some reading I thought I could use an op-amp in a voltage follower configuration.

Considered to use the TI TLV2460 family but seemed like the output current goes to zero when powered with a 5V supply with a near 5V output voltage.

Seems like all the op amp are like this. What can I do? I need 60 mA of output current from .25mV to 4.9V.

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  • \$\begingroup\$ Is your sensor single-ended (i.e., is the other end grounded, and it always draws current)? \$\endgroup\$ – TimWescott Aug 23 at 21:39
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Many opamps have a max current in the 30mA to 50mA, especially ones that are rail to rail. 60mA is hard to find, and opamps with a max current of more that 200mA are very few.

The best way is to use a rail to rail opamp in a voltage follower configuration. Because the op amp can introduce errors, select one that has a low input voltage offset. Either set the voltage rail above the range of the DAC to avoid common mode problems, or get a rail to rail opamp. Maybe paralleling the ADA4522 might work.

Or you can parallel any op amp:

enter image description here Source: How to increase the rated output voltage/current by connecting multiple op amps?

OR like this:

enter image description here
Source: https://www.analog.com/en/technical-articles/paralleling-amplifiers-increases-output-drive.html

Paralleling can increase current output, however, series resistors are needed on the output of the amp which can introduce error and require analysis to see if it will fit your application.

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If your load is grounded you could add a BJT to a single-supply op-amp as below.

schematic

simulate this circuit – Schematic created using CircuitLab

The Zener and C1 and R3 form a 7.5V supply for the IC to allow it to drive the base of the transistor to 5.6V or so.

enter image description here

As shown this typically won't go all the way to 4.9V quite, but close. If you really need the extra 50-100mV you can use a MOSFET source follower rather than the BJT but you might need a bit more supply voltage for the op-amp or to use a more expensive rail-to-rail output type.

Also note that this is not short-circuit proof as the op-amp outputs generally are. If that is a requirement, you could add a resistor in series with the collector and connect it to a higher voltage (perhaps 9V) supply and ensure it's got enough dissipation capability to handle a short (probably a beefier transistor would be required).

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  • \$\begingroup\$ I only have a 5v rail in my design. That will be Both the input and supply. \$\endgroup\$ – Peter Ruggiero Aug 23 at 19:30
  • \$\begingroup\$ You said it was automotive- no 12V? \$\endgroup\$ – Spehro Pefhany Aug 23 at 19:31
  • \$\begingroup\$ When I look at these op amp datasheets for a 5v supply at a 5v output it shows the output current is 0. If that's the case, how can I ever get what I need? \$\endgroup\$ – Peter Ruggiero Aug 23 at 19:49
  • \$\begingroup\$ No only 5v goes to this sensor and is available \$\endgroup\$ – Peter Ruggiero Aug 23 at 19:49
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    \$\begingroup\$ Solutions to get lots of current with just 0.1V drop from the supply can get pretty cheezy -- are you sure you can't supply your circuit with 12V? \$\endgroup\$ – TimWescott Aug 23 at 21:41

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