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High Supply Currents and Output Oscillations on the LTC6269 as Unity Gain Amps

Category: Datasheet/Specs
Product Number: LTC6269

Hello,

We plan to use the LTC6269 in one of our application boards. At the moment we are trying to familiarize ourselves with it.  The main purpose is to use one channel as a TIA, while the the other one will be configured as a buffer. Here is our application schematic. An 'X' on the component value means the part isn't soldered, and it is only a spacer component.

However, we are seeing 2 issues with it. One is very high supply current, at 60mA. The second one is oscillations at the output. We think the two are related.

To debug these, we modified the existing board by changing resistor values and built a test circuit where we have configured both op-amps as unity-gain (buffer) amplifiers, with the +IN on each channel connected to GND. This was to keep the circuit as simple as possible. The idea was to get it to work properly with this basic configuration. If the issues still persist in these simple configurations, then there is no way more complex configurations can be made successful. Here is a schematic of our test setup. V+is at 5V, and V- is at GND. There are no resistive or capacitive loads at the outputs. The opamp channels were made independent of each other.

However, even with the simple connection above, the supply current drawn was still at 60mA. This is too high. The datasheet only indicates quiescent current of 16mA per amplifier, so we would expect probably 30-40mA at this configuration for this dual-channel device. This high current has been observed using multiple devices. This was verified on multiple boards too. 

We also did another experiment where we forced a voltage on +IN A, while keeping +IN B at GND. We expected to see OUT A following +IN A. We still saw oscillations at the output, and the supply current remained high. Here are some plots. 

OUTA and OUTB are clean when +IN B = GND, +IN A = 0V; Isupply = 60mA        [Test Conditions: V+=5V, V-=GND; Channels A and B in unity gain configuration, both channels independent of each other]

The plot below is on OUTA. OUTB is not shown, but looks the same. 

  

With the other conditions set the same, we increased +IN A to 100mV. Oscillations are seen on OUTA as shown below. OUTB is again not shown, but remains clean as in the first plot.

Below is another plot where +IN A is at 1V, with +IN B still at GND. 

We have tried to think of possible reasons why we are seeing such high currents and oscillations even when we connected the op-amps in unity gain, but we could not think of anything electrically that could be causing this. The V+ supply is clean. We have put a scope on it and confirmed it is clean. The +IN and GND nodes are clean as well. These buffer configurations are supposed to be simple circuits, but we could not get them to work. Hence, we are reaching out to ADI regarding the use of the LTC6269. Any help woúld be much appreciated.

In case anyone is interested in looking at the PCB layout, here is a snapshot of it. 

Thank you!

  • Hi iansacares:  The LTC6268/9 is a very high frequency amplifier.  It doesn't like looking into 0 Ohms or open, either at its +input or on its output.  Try making the input and/or output resistors somewhere between 10 and 100 Ohms, even though the other end is ground or open.  It's usually a reflection thing.

  • Hello Glen, 

    Thanks for the tip. I tried to use 100ohm resistors as input and output resistors. The oscillations have dropped, but they haven't totally disappeared. Also the supply currents are still high. They remain around the 60mA level. Here is a scopeshot from the output of the unity gain amp, with 1V at the +IN. It has improved a lot vs. the initial one, but still it is there. 

    I've experimented more by increasing the output resistor to 1Kohm, and adding a 10pF cap to GND after it to build an RC filter at the output. I could clean up the signal, but I think this shouldn't be necessary when using the LTC6269. However, perhaps I am also wrong.  Let me know if output filtering is recommended for this device. Here's a scopeshot of the "clean" output with 1V again at +IN.

  • Hi,

    You may not have proper grounding, the decoupling capacitors may have higher impedance path to the supply pins than required by such a high bandwidth amplifier. At least the negative supply pin seems to have rather thin and lengthy connection to the ground plane via a resistor, so as to the decoupling capacitors. The other layers are not shown. Be sure to use proper supply, this can also be a source of problems. I would use a regulator close to the opamp with proper input and output filtering/decoupling.

    I think the possible grounding/decoupling/supply problem is also made probable by the signal shown: you have about 8Vpp signal at the opamp output, while the supply voltage is only 5V.

    As a matter of fact, I can’t see reason to use a 0.5GHz amplifier when the signal bandwidth is limited to 1kHz by the active filter, the photodiodes do not seem to require high bandwidth too. You would have much less trouble if you would choose a more suitable opamp, see the very useful Photodiode Circuit Design Wizard provided by ADI.

    Zoltan

  • Hello Zoltan, 

    Thanks for your inputs. We will consider your suggestions and have a look at these. 

    Regards, 

    Christian 

  • Hi Christian ,
    One more thing. I don't know what your DC source was for the tests, but you may try to use a small resistor divider using say 1k-1k resistors between the supply pins of the opamp to halve the supply and connect it to the input of the opamp. You can use a decoupling capacitor between gnd and the input too. Put it very close to the opamp. Then you may have less chance to introduce longer loops that can also be among the reasons.

    The oscilloscope probe means some capacitive load, so put a 50 Ohm or similar resistor close to the opamp output and connect the probe to the other terminal. 
    Zoltan

  • Hi,

    I believe that the questions have already been answered so I am closing this thread. Thank you!

    Regards,

    Mae