Combining digital signal (0/24 VDC) with analog signal (0-10VDC)

Question

I need some hints how to make an interface circuit that would combine two signals into one:

• one is a Digital output is used to current sink (manufacturer wording) a load that is from 5V to 24V with a max current of 250 mA. This give a square like signal. Maximum frequency is 5 MHz. I refer to this as the digital signal or pulsed signal.
• programmable analog output, from -10V to +10V, at 16 bit resolution. I need only the 0-10V part. I refer to this as the analog signal.

The output of the combined signal are used to control the power settings in a machine and if it is on or not. This is done with one input at the machine accepting 0-10V as a control signal. 0V is zero power and 10V 100% power. The digital on/off signal determines if the voltage level set by the analog output is turned on or off at the machine. At the moment we are using a frequency of approx. 5 kHz on the digital signal.

The interface circuit is not driving the machine.

I have present a 24 VDC supply and 5V DC (500 mA) to drive the interface circuit.

I have been looking at using a operational amplifier, like the AD712, and a switch, like the ADG202A, to switch the operational amplifier on and off, is this a good solution?

As mentioned the analog output can be negative, but the machine should only be controlled with between 0-10V. Would the best place be to set a diode before the operational amplifier, as this mean the amplifier only handles positive voltages, and the interface circuit protect the machine from programs errors that would put negative voltages on the analog output?

I am sorry for my bad english and punctuation!

The analog output can provide 5 mA (2K Ohm) according to the documentation. The control input at the machine does not specify anything else than "control signal 0-10V"

Yes, this is my project on RentACoder.

Answer 1

As the comment says, you can get a much better answer to this question if you give us some more information. One thing that's not clear is, are the signals your talking about actually meant to convey power to their receiver, or just control power that's already available there.

I'll assume these are just control signals.

I can think of two simple ways to accomplish what I think you want.

1. Simply sum the two signals. Because the digital signal has such a high magnitude, you'll have a signal between 0 and 10 V when the digital value is low, and between 24 and 34 V when the digital value is high. At the receiver, use a comparator with a 17 V threshold to extract the digital value, then subtract it off to recover the analog value. You will likely have some glitches in the recovered analog value whenever the digital value switches -- how you deal with those will depend on details of your requirements that you haven't shared.

2. Modulate one or the other of your two signals onto a high-frequency carrier. For example, use the digital signal to switch a (for example) 100 kHz sine wave onto your signal line. (I'm assuming your analog signal is only changing slowly. If your analog signal has high frequency content, then the details of how to do this will depend on what are the frequencies in the analog signal). At the receiver end, use a low-pass filter to extract the analog signal; and a high-pass filter and a rectifier to extract the digital signal.

Edit

Your edit helps quite a bit. It sounds like you can just use a switch like the ADG202A you mentioned, with no other op-amps or anything needed. Use the digital signal to control the switch. Route the analog signal through the switch to the machine under control. Use a pull-down resistor to pull the control voltage to 0 when the switch is opened. If you are worried that the analog voltage might be accidentally programmed below 0 and this could damage the machine, you could add a diode between the control pin and ground to prevent the control voltage dropping (much) below ground.

Some things you still need to explain to know if this will work: How much current can your programmable analog source generate? How much current is required at the control input of your machine?

** Edit 2 **

Here's a diagram of what it sounds like you want:

This will set the machine power to 0 when the digital signal is 0. If you want it the other way, you can use the ADG201A NC switch instead.

The ADG202A has a fairly high 60-Ohm on resistance. If the machine draws significant current at the control input you may need to locate a switch with lower on-resistance, or buffer the signal with an op-amp between the switch and the machine.

Answer 2

As I understand it, you have a 0-10V signal that is supposed to proportionally control the power to some device, and a digital signal that is supposed to enable/disable it. When the digital signal is set to enable, the power is controlled according to the analog level. When the digital signal is at disable, the power to the device under control is off regardless of the analog level. What you want to do is create one composite signal that can transmit both pieces of information.

If the above is not correct, then you need to fix your question. If the above is correct, then the obvious answer is to override the analog signal with 0V when the digital signal is indicating disable. This is so obvious as to put in doubt whether that is what you are really asking about. A simple way to achieve this is to put some resistance in series with the analog signal, then clamp that to ground when the digital signal indicates disable, and leave it alone when enabled. A NPN transistor of N channel FET can easily perform this clamp-to-ground function. The transistor would be turned on in the disable state and off in the enable state.