Products > Test Equipment
AWG Buffer Amp
mawyatt:
Been using the Juntek DAP-1698 Dual Channel Power Amp as a AWG buffer, and thought others might be interested in a low cost Dual Channel Low Frequency AWG Buffer.
The design is based around a TI LM1875T power Op-Amp which is direct coupled input and output and has dual 26V power supplies from a XL6012 DC to DC converter and built-in fan which is noisy! Input power is 12VDC and supplied with a 6A AC to 12VDC converter. The LM1875 is a low frequency high power type Op-Amp that has a GBP, power BW and Slew Rate, 5.5MHz, 70KHz and 8V/us. The LM1875 design is a class A/B linear type output with overcurrect protection on both +- output current. It's heavily biased into the Class A for improved distortion, at the expense of static power dissipation with a nominal bias of ~70ma. Static PD is ~3.6 watts. Check out the data sheet on the TI LM1875, looks like a nice IC to base this around.
We just ran into an issue with the DAP-1698 going into a thermal limit cycle on both channels and suspected that the LM1875 was getting too hot because of an improper heatsink cooling. After removing the PCB, the two LM1875 were not mounted securely to the heat sink, causing a high thermal impedance and allowing the IC core temperature to rise and initiate thermal shutdown cycle. So the LM1875 IC did what it was designed to do and protect itself from over temperature :o
Once back together the DAP-1698 was fine driving a 100 ohm load at 40Vpp square wave or sine wave. Attached are some images and screen captures of the 40Vpp pulse rise and fall, 1, 10 & 100KHz sinewave and a 200KHz showing slew rate limiting. Also included a FFT of the output at 10KHz. Later maybe some tests at higher load currents if I can find some loads.
With the input shorted, the output measures ~20mvpp noise, ~2.5mv SD and ~6.5mv DC offset.
Anyway, hope this helps anyone looking for an inexpensive way to expand their AWG in voltage and current capability at DC & lower frequencies.
Best,
Edit I can confirm that the amp can deliver a 36vpp square wave into 33 ohms, that's ~ 9.8 watts!! Also can deliver ~0.9 amps peak before current limit kicks in.
mawyatt:
Another image and the waveforms mentioned above.
Best,
mawyatt:
Since there has been discussion of the value of a simple AWG on a DSO, just did a quick Bode Plot of this amplifier using the DSO built-in AWG.
Here you can see the amplifier voltage gain of ~6dB. Note how the gain slightly peaks, indicative of a underdamped amplifier response which shows in the step response with some overshoot. The -3dB bandwidth extends to ~1MHz for a 1vpp input (2vpp output) and ~500KHz for a 3vpp input (6vpp output), 3vpp was the max allowed input for the DUT from the Bode Plot using the DSO AWG.
For a check with an external AWG, the Bode' plot limits the max DUT input to 6vpp (12vpp output), this produces a -3dB bandwidth of ~250KHz, which is respectable considering the spec of 100KHz and amp cost.
Best,
ExaLab:
Interesting product, even for its price.
Can you check if the BNC input of the device is properly decoupled from the LM1875?
This is a fundamental trick in order to avoid the damage of the connected instrument (that may be an expensive function generator...) in case of failure of the LM1875.
It would be great if you had the wiring diagram of one of the amplifier stages (it should be really simple).
Just to take a look before purchasing...
mawyatt:
Don't have a schematic, but looks like they have a 2X attenuator on the input with 10k resistors. This would explain the LM1875 set up as a 4X amplifier with a phase margin of ~45 degrees and a gain margin of ~6dB, so the overall effect is a gain of 2X input to output for the DAP-1698. The LM1875 would be very underdamped with a closed loop gain of just 2X (6dB).
We required a higher voltage than the DAP-1698 can provide for some applications and began development of custom AWG amplifiers capable of suppling 100~120Vpp based upon the LM3886 and TDA7293. So can see more detail on this thread (also one with a 150~180Vpp output at low currents). We'll post information on these devices later.
https://www.eevblog.com/forum/testgear/whats-inside-a-feeltech-fpa-301-better-than-juntek-dpa-2698/msg3307300/#msg3307300
Best,
Navigation
[0] Message Index
[#] Next page
Go to full version