Operational amplifier design in high performance analog front end .

High-speed conversion systems, especially in the telecommunications field, allow analog-to-digital converter (ADC) input signals to be AC-coupled (by utilizing transformers, capacitors, or a combination of both).but fortestandMeasurementIndustry-wise, front-end design is not so simple because, in addition to providing AC-coupling capability, this application area typically requires that the input signal be DC-coupled. Designing an active front end that provides good impulse response and low distortion performance (≥500MHz DC frequency) is challenging. This article provides several design ideas and suggestions for analog front ends used in high-performance ADCs for high-speed data acquisition.

Operational amplifier design in high performance analog front end .

Figure 1: LMH6703 frequency response.

Using a differential amplifier is the preferred method for connecting high frequency analog signals to the input of the ADC. Therefore, the first device to choose is a differential output op amp. There are two main considerations when choosing this type of device: the gain-bandwidth product and the ability to set the common-mode output voltage of the op amp from an external voltage.This is becausedriveIt is important that the signal amplifier of the ADC input sets the common mode output voltage (VCMO) within the optimum ADC range. If these conditions are not met, the performance of the ADC will degrade significantly as the mismatch between the amplifier’s VCMO and the ADC’s optimum input common-mode voltage increases.

The main disadvantage of wideband differential op amps is that their gain is usually limited and its gain level may be preset internally. Depending on the application, it may be necessary to add a preamplifier to the design to meet the necessary gain requirements.

As for the preamplifier, a wideband op amp should be used to meet the expected input frequency of the ADC. For systems with sampling rates up to 1GSPS, this is equivalent to requiring an oversampling system with up to 500MHz of input bandwidth.

Figure 2: Secondary amplifier circuit diagram.

For an op amp that works with a large gain (eg, AV=10) and can maintain such a large bandwidth, it is equivalent to a 5GHz gain-bandwidth product (GBW). Most voltage feedback amplifiers cannot meet this requirement due to the direct trade-off between frequency response and gain inherent in this architecture. However, current feedback amplifiers maintain a good relationship among these parameters because their performance is usually determined by the value of the feedback resistors within the op amp circuit. The operational amplifier LMH6703 is well suited for high bandwidth operation with gain settings of 1 to 10. The device can be used with selected differential amplifiers to provide additional gain requirements in high bandwidth systems such as oscilloscopes and data acquisition cards. The frequency response of this amplifier is shown in Figure 1.

Figure 3: System frequency response with extended AC signal performance.

If the gain is set to 10 and the bandwidth is 500MHz, the recommended feedback resistor of 300 ohms is obtained from Figure 1 (RF1).

So RG1 (Gain Resistor) can be chosen to be 33 ohms. Figure 2 is an example of a circuit using the LMH6703 with a differential amplifier.

In addition to systems requiring fixed gain levels with appropriate DC signal paths, this application also requires an AC coupling mode. This is because the DC signal path is usually limited by the gain bandwidth produced by the input amplifier.For data acquisition devices or those requiring very wide input bandwidth and low distortioncommunicationIn terms of channels, we need to use the AC signal channel. This extends the upper limit of the input frequency beyond the capacity of the DC signal channel.

There are many solutions, and the choice of which method to choose depends greatly on the minimum input frequency and the desired high frequency performance. For the highest AC performance at high frequencies (≥200MHz), baluns provide a solution for single-ended-to-differential conversion because the added signal distortion is minimal. The tradeoff is that baluns are lossy devices that attenuate the signal by a small amount (-1~2dB), and they have poor low frequency performance. By using single-pole RFrelayTo switch a single-ended output signal from a preamplifier to a differential amplifier or a balanced/unbalanced conversion circuit, a balanced/unbalanced coupled signal path can be inserted into the circuit shown in Figure 3. Another SPDT RF relay is required to forward the output of the balun and differential amplifier into the ADC input.

Figure 4: 198 MHz sine wave (sent by high speed differential output op amp, sampled at 500 MSPS by ADC08D500)FFT map.

This circuit is well suited for high-end test and measurement equipment. However, for cost-sensitive applications, the cost of RF signal relays can tax the system budget, especially if multiple channels are required. It is therefore advantageous for low-speed systems to choose differential output op amps that can be used in both AC-coupled and DC-coupled modes, eliminating the need for balanced/unbalanced conversion circuits. Amplifiers that are particularly suited for this task are starting to emerge, with progressively higher performance in terms of bandwidth and THD.

For an 8-bit 1GSPS converter, a differential amplifier capable of delivering -50dB THD at 500MHz with a minimum bandwidth of 1GHz is suitable.Utilize off-the-shelf op amps that dramatically reduce front-end design timeelement, you can get better dynamic performance from the high-speed ADC. At the upper frequency limit, the SINAD loss caused by the amplifier does not exceed 3~4dB. Figure 4 shows the FFT of a 198MHz input signal (buffered by a wideband differential output amplifier and sampled at 500MSPS by an 8-bit ADC). The graph shows that the amplifier has very low 2nd and 3rd order harmonic distortion at this frequency, so that the noise and distortion figures of the signal captured by the ADC are comparable to the performance obtained from a dedicated AC-coupled signal path.

Summary of this article

Amplifier performance is continually being improved to increase bandwidth and reduce THD.As ADCs move into the GSPS range, we need to be able to communicate with these convertersinterface‘s amplifier. Eliminating circuit channels not only reduces system cost without sacrificing system performance, and allows designers to achieve higher performance at lower cost while reducing front-end circuit design time.

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