Analog Output with jumper settings for voltage and current ranges

PWM@ Analog to Digital@ Digital to Analog@

Lots of people have asked for it so here it is, a little analog output circuit for microcontrollers that covers the common voltage and current ranges used in instrumentation.

The input can be, for example, 0-5V analog from a DAC, or 0-5V PWM signal. The 100k resistor and 1uF capacitor filter the PWM signal to make it smooth DC. With J3 ON the input becomes 1-5V to produce the offset zero ranges.

The first Op Amp is the output driver, and its feedback path can either be voltage (J2 ON) or current (J4 ON). With J1 OFF the Op Amp gives a gain of 1, and with J1 ON it gives a gain of 2.

With J4 ON, the second Op Amp provides current feedback for the first Op Amp. It acts as a differential amplifier, using the 100R resistor as a current sensing element. The 47R resistor does nothing except fix a linearity problem the circuit had at low outputs. Two 10k resistors were used in series because 20k was not available.

The jumper settings indicate how to select the desired voltage and current ranges. Hope the ideas in this circuit are helpful to you all.

Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Mobile: 027 433 4069
eMail: @spam@brent.brown at clear.net.nz

Comments:

• You may expect more stable movement by giving a capacitor of about 500PF between the negative input and output of two OP amplifiers.
• Brent: Yes that's a good idea. It would be especially necessary if the circuit is required to drive cables of any significant length.+

Questions:

• edm_1968 asks: "If I wanted to replace J2 and J4 with a digital input from a uC, could I use a NPN transistor? Or PNP? Thanks very much for the cool circuit."
• Brent: A transistor would not be ideal, consider using a CMOS bilateral switch instead eg CD4066.+
• What would be a good PWM frequency to have a smooth DC output in this case?
• James Newton of Massmind replies: That depends entirely on the size and electrical characteristics of the motor.
• Brent: The RC network (100k resistor & 1uF capacitor) is the simplest possible filter and has only modest performance. Time constant is given by R x C = 0.1 second. The higher the PWM frequency, the lower the amplitude of the ripple that will appear on the ouput.

Comments:

• Thank you very much. i was searching this for a long time. Thank you.+

Questions:

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