LT6600-5 Gain Profile

I'm currently using the LT6600-5 for a current sense resistor application, however and having some trouble with the design producing a wildly different gain than expected. The intended operation is for the LT6600 to amplify the measurements from a sense resistor with a gain of 18dB and measurement bandwidth of 5MHz.

The following is the schematic I've designed for this application. Gain resistors of 100R are used, along with 82pF parallel capacitors for compensation (I've tested with and without these populated). In addition a high frequency ferrite bead (Murata BLM15BD121SN1) is added to reduce HF noise pickup, this should have little impact on the gain, if anything causing some attenuation above 4MHz. The Vocm voltage is driven from a stiff 1.024V supply to match the requirements for an ADC which follows. The Vmid input is simply using the internal divider with a bypass capacitor.

Amplifier Circuit

When simulating this design in LTSpice, the results are quite acceptable, not quite flat due to the compensation, but good enough for our application:

LTSpice Simulation

However measurements of the actual circuit are showing a widely different gain response, whereby the gain starts around the intended 18dB gain, but then rises rapidly to 25dB by 5MHz, which seems very strange.

Measurement Results

I'm trying to work out the possible causes of this gain response. In the simulation I'm able to force similar results by increasing the compensation capacitors to around 600pF which is far higher than in the actual circuit. In fact I've tried removing the compensation capacitors entirely but get the same results.

Is there anything that I may have missed in the circuit? Am I operating the LT6600 outside its specification?

  • 0
    •  Analog Employees 
    on Aug 5, 2020 11:46 PM 3 months ago

    Hi,

    I have contacted the responsible Apps Engr of this part.
    Will provide feedback the soonest.

    Thanks and regards.

  • 0
    •  Analog Employees 
    on Aug 8, 2020 6:31 PM 3 months ago

    The LT6600-5 at 18dB passband gain will have a peak gain around 5MHz higher than the 18dB at lower frequencies.

    The LTspice model is not adequate for high gains because the simple model amplifiers do not show the limits of the IC amplifiers.

    At 12dB or less passband gain the LT6600-5 will have less gain peaking than at 18dB gain.

    However, the LT6600-5 is fully differential 4th order filter circuit and is not the best choice for a current sense input.

    Consider a high-speed op amp like an LTC6228 as a current sense amplifier.

    Best regards,

    Philip Karantzalis

    Senior Apps Engineer.