To reliably capture high-frequency signals and fast transients, wideband data acquisition systems such as oscilloscopes and active probes require a high-performance analog front-end (AFE) signal chain that can:

  • 1 V PP signal is supported (at least) to ensure high signal-to-noise ratio.
  • Supports high input impedance (Hi-Z) from DC to 500 MHz to prevent DUT loading.
  • Provides low noise and distortion to maintain high signal fidelity.
  • Provides high DC accuracy.

One way to overcome these design challenges is to create a composite loop-based approach that interleaves low-frequency and high-frequency signal chains for DC accuracy and wide signal bandwidth.

ASIC-Class Performance Using the BUF802

Learn how to more reliably capture high frequency signals and fast transients in wideband acquisition systems in the video "BUF802: Wideband, High Input Impedance JFET Buffer."

Given the complexity of implementing composite loop-based circuits that meet system requirements, engineers often must design custom application-specific integrated circuits (ASICs) or use multiple discrete components, as shown in Figure 1. Both options have drawbacks, including the need for ASIC expertise and additional design complexity. Both approaches also have trade-offs in performance and cost: discrete implementations are cheaper than ASICs, but cannot match their performance levels.

Figure 1: Discrete Buffer Composite Loop with Precision Amplifier AFE

In this article, I will explore the design challenges of discrete buffer composite loop implementation versus single-chip implementation using the BUF802 Hi-Z buffer.

Discrete Buffer Compound Loop Architecture

The discrete implementation of the Hi-Z AFE in Figure 1 uses a precision amplifier and a discrete junction field-effect transistor (JFET)-based source follower circuit, configured in a composite loop. The loop splits the input signal into low-frequency and high-frequency components, brings these two components to the output through two different circuits (transfer functions), and recombines them to reproduce the net output signal, as shown in Figure 2.

Figure 2: Discrete composite loop low and high frequency paths

The low frequency path provides good dc accuracy for the net transfer function, while the high frequency path based on the JFET source follower allows the net transfer function to have a wide signal bandwidth with low noise and distortion. One of the main challenges of the circuit shown in Figure 2 is to achieve smooth interleaving of the two paths to ensure a flat frequency response. Any mismatch in the transfer functions of the two paths results in a discontinuity in the net transfer function frequency response, resulting in a loss of signal fidelity.

Goals of the Compound Loop Architecture

At DC or low frequencies, the C HF (high frequency capacitor) is open and the voltage output (V OUT ) is controlled by a precision amplifier in the low frequency path. The ratio of alpha and beta resistor networks controls DC or low frequency gain.

At high frequencies, C HF is shorted and the precision amplifier runs out of bandwidth with a finite gain-bandwidth product. Discrete buffers are used as JFET sources, and negative positive and negative emitter followers determine V OUT. A discrete buffer stage called gain (G) in Figure 3 determines the high frequency path gain.

Figure 3: Discrete Buffer Compound Loop Architecture

At mid frequencies, since both the low and high frequency paths determine the output, careful tuning of the individual gains and the interaction of the poles and zeros is important to ensure a flat frequency response. Achieving mid-frequency gain equalization is challenging because the same components, C HF and R HF (high-frequency resistors), determine the poles of the low- and high-frequency paths, as shown in Figure 4.

Figure 4: Discrete buffer frequency response

The composite loop should have a flat frequency response and high crossover frequency region for low 1/f noise and fast overdrive recovery.

Complexity of discrete implementations

Given the interdependence of the low and high frequency paths, as shown in Figure 5, the values ​​of CH and CF (compensation capacitors) are in the order of tens of nanofarads to achieve a flat frequency response. But these values ​​result in a crossover frequency range of tens to hundreds of hertz, which limits the DC noise performance of the signal chain.

Figure 5: Interdependence of low and high frequency paths

Another challenge in discretely implementing the composite loop is that the poles of the precision amplifier open-loop gain and the poles of the resistor-capacitor network of R HF and CH HF contribute to the two-pole network in the low-frequency path, resulting in instability. Implementing an additional network (labeled as a gamma network in Figure 3) on the precision amplifier will compensate for this instability, but requires tuning to achieve a flatter frequency response, further adding to the complexity of creating a smooth frequency response. operating range.

Use BUF802 to realize compound loop

Since one of the main limitations in implementing a discrete composite loop is the interdependence between the low and high frequency paths and the need for an additional gamma network to compensate, the BUF802 has an auxiliary path inside the device. Connecting the output of the precision amplifier to the auxiliary path creates a composite loop while ensuring isolation between the low and high frequency paths. Isolating different frequency paths creates higher crossover frequency regions and eliminates gamma networks and compensation circuits. The low frequency and high frequency signal components are recombined inside the BUF802 and reproduced at the OUT pin, as shown in Figure 6.

Figure 6: Composite Loop Precision Amplifier with Internal BUF802

in conclusion

Integrated Hi-Z buffers such as the BUF802 help solve complex loop-based implementation challenges. Integrated protection features such as the BUF802's input/output clamping help protect subsequent stages in the signal chain, reduce overdrive recovery time and input capacitance, and improve system reliability.

When considering AFEs for today's applications, you must also keep in mind future measurement needs, which often require additional bandwidth. This bandwidth can greatly improve measurement accuracy and ensure that system design investments remain relevant to future test requirements.

Reviewing Editor: Fu Ganjiang

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