Tool/software:
We are looking for a solution to increase the driving signal sources 0 to 10v (with resolution 0.01 steps) from 4mA to 80mA. So, the output signal should look like the same with input signal voltage but in 80mA.
i.e.
input signals : [0 -10V] , 4mA
Required Out Signal : [0-10V]linear , 80mA
Basically an linear voltage with current amplified up to 80mA .
Kindly check and help with this.
Yes, A device like ALM2402-Q1 or OPA593 could be suitable for this voltage buffer application.
As Clemens has pointed out, you will need a V+ greater than 10V, and a V- less than GND to accurately create the desired voltages at 80mA.
Using something like 12V or 15V V+ supply would give you plenty of V+ headroom for output swing for OPA593.I would recommend a V- close to -2V to accurately set the output to GND at 80mA.
What is this output buffer connecting to? The power amp will only source/sink the desired output current required by the load.
Thanks,
Jacob
Hi Vishnu,
When you say protection for the FPGA from the OPA593, this is likely unnecessary.
We will be driving the non-inverting pin of the OPA593 which has a input impedance of 10Gohm
This means the FPGA card will not need to be concerned with being damaged from the OPA593 as there is no ability for the non-inverting terminal to generate current significant flow.
The more concerning problem involves potentially having the FPGA card output a signal when the OPA593 is unpowered.
Will the OPA593 ever be turned on after the FPGA? If this is possible, then we may need to include external diodes and current limiting resistors in series with the non-inverting input
If you want 4 OPA593 devices on one board, you need to consider the (Iq of the device + the maximum sourcing current in the application) * safety factor
=( 4mA + 80mA ) *4 * safety factor
I would shoot for at least 500mA to be safe, but you can certainly cut this back if you can tolerate less margin.
Thanks,
Jacob
Hi Vishnu,
We do not make a board with 4 OPA593 devices installed, but this would be very easy to do if you have access to a PCB design tool.
Alternatively, you could purchase 4 OPA593EVMs, but this may be a bit bulky in size.
The device uses a bottom mount thermal pad, so we use the internal copper layers of the PCB to act like a heatsink. You can see we connect the heatsink to V- in the OPA593EVM for enhanced thermal performance:
What supply rail is the FPGA card using?
Ideally, this is were we will connect our protection diodes to.
Thanks,
Jacob
Hi Vishnu,
All of our EVM's will approximately be the same size of the OPA593EVM. This is done to enhance the configuration options when evaluating the device.
Why not create a custom PCB with 4 OPA593 or 2 ALM2402Q1 devices on board? This would be much smaller form factor, and have the features you seek.
1. Both solutions would work with +-12V
2. OPA593EVM has an external reverse polarity protection diode on the supply rail here:
And external output clamping diodes here:
This EVM does not have a place for external input protection diodes. I would recommend using a custom PCB with diodes to the FPGA supply to ensure no overvoltage events occur while the power amp is turned off.
ALM2402Q1EVM lacks external protection diodes on the output and input.
3. Board modifications would involve moving jumpers around and possibly changing two component values to set G=1V/V. These board modifications would still fail to solve the lack of external input protection diodes. Understand that these diodes should not normally be used. If you expect there to be use cases where the FPGA could be on and the power amp could be off, then it is important to have these diodes.
Best,
Jacob
Hi Vishnu,
I have to make a correction to my previous statement. When I said +-12V, I meant to say ALM2403-Q1 not ALM2402-Q1. ALM2403-Q1 will have the supply capacity to use 24V = +-12V.
We can use +12V and -3.3V if we want to use ALM2402-Q1 though.
ALM2403-Q1 is very similar to ALM2402, just with many parameters upgraded. The pinout is similar, but more clear with how to connect supply pins:
pins 10-12 will be +12V, pins 6 and 14 will be V- for ALM2403-Q1. Thermal pad can connect to -12V or electrically floating.
What accuracy do you need for the buffer amp? ALM2402Q1 has a typical Vos specification of +-1mV, and a max of +-15mV:
ALM2403-Q1 has a typical vos specification of +-6mV and max room temp Vos of +-25mV
Is this amount of error tolerated in your system?
Best,
Jacob
> If opamp is not powered how/which is the reverse current flow path ?
Output current from FPGA can cause the internal input ESD diodes to forward bias when the amplifier is off. This current then typically flows to the floating supply rail of the DUT. This is therefore not a revers current flow, but a forward current flow. There is no concern for revers current flow in this application.
This application note covers this topic well: Op Amp ESD Protection Structures (Rev. A)
Fortunately, if your FPGA can only source 4mA,so we should be fine. Even if the amp is unpowered, we are still under the 10mA input current limit for this device:
If you want, we could still add some input current steering diodes here to be extra safe.
The important diodes to add here are the protection Schottky didoes which will prevent damage during reverse voltage connection. These diodes will also prevent back feeding current during fault condition: protects voltage regulator.
If you want to add TVS clamping diodes to the supply rail between V+ and GND and V- and GND, this can also help protect the DUT during short to voltages above or below supply rail. If you do not believe this condition is possible, then it is likely unnecessary.
Finally, you can add output clamping diodes to the rail. This will help steer current away from the output pin during over-voltage events. This is another optional inclusion which may be completely unnecessary depending on what you expect the power amp output to connect to.
Here is a great article that covers protection schemes for power amplifiers: Overvoltage Protection of Resolver-Based Circuits
I have created a demo circuit with the ALM2403-Q1, but this same circuit could be modified to use ALM2402-Q1 or ALM2403-Q1with +12V and -3.3V
Best,
Jacob
Hi Vishnu,
Thank you for the details on DUT connection. Correct, extra protection is not needed here.
What is the max error you can tolerate in your system? +-6mV is typical error for the device. This can go up to a max of +-25mV at room.
You are correct on your fault isolation. No need to use external input diodes here.
Thanks,
Jacob
