The upper curve is for a total 2 tone envelope of 4vp

This preview shows 10 out of 12 pages.

The upper curve is for a total 2-tone envelope of 4Vp-p, requiring two tones, each at 2Vp-p across the OPA687 outputs. The lower curve is for a 2Vp-p envelope requiring each tone to be 1Vp-p. The basic measurement dynamic range for the two close-in spurious tones is approximately 85dBc. The 4Vp-p test does not show measurable 3rd-order spurious until 25MHz, while the 2Vp-p is ummeasurable up to 40MHz center frequency. Two-tone, 2nd-order intermodulation distortion was unmeasurable for the circuit on the front page of this data sheet. (= 1.90pF) (= 43.8pF) (= 121MHz) FIGURE 4.Broadband Low Inverting Gain External Com- pensation. R F 850 C S 44pF OPA687 +5V –5V V O V I C F 1.9pF R G 200
Image of page 10

Subscribe to view the full document.

11 OPA687 SBOS065A LOW NOISE FIGURE, HIGH DYNAMIC RANGE AMPLIFIER The low input noise voltage of the OPA687 and its very high 2-tone intercept can be used to good advantage as a fixed- gain IF amplifier. While input noise figures in the 10dB range (for a matched 50 input) are easily achieved with just the OPA687, Figure 5 shows a technique to reduce the noise figure even further while providing a broadband, high gain IF amplifier stage using two stages of the OPA687. This circuit uses two stages of forward gain with an overall feedback loop to set the input impedance match. The input transformer provides both a noiseless voltage gain and a signal inversion to retain an overall non-inverting signal path from P I to P O —since the 2nd amplifier stage is invert- ing to provide the correct feedback polarity through the 6.19k resistor. To achieve a 50 input match at the primary of the 1:2 transformer, the secondary must see a 200 load impedance. At higher frequencies, the match is provided by the 200 resistor in series with 10pF. At lower signal frequencies (f < 80MHz), the input match is set by the feedback through the 6.19k resistor. The low noise figure (5dB) for this circuit is achieved by using the transformer, the low voltage noise OPA687, and the input match set by feedback. The first stage amplifier provides a gain of +15. The very high SFDR is provided by operating the output stage a low signal gain of –2 and using the inverting compensation to hold it stable. Depending on the load that is driven, this circuit can give a 2-tone SFDR that exceeds 90dB through 30MHz. Besides offering a very high dynamic range, this circuit improves on standard IF amplifiers by offering a precisely controlled gain and a very flexible output load driving capability. DESIGN-IN TOOLS DEMONSTRATION BOARDS Two PC boards are available to assist in the initial evalua- tion of circuit performance using the OPA687 in its two package styles. Both of these are available free as an unpopulated PC board delivered with descriptive documen- tation. The summary information for these boards is shown in the table below.
Image of page 11
Image of page 12
You've reached the end of this preview.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern