Op Amp Design in High-Performance Analog Front Ends

High-speed conversion systems, especially those 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 for the test and measurement industry, front-end design is not so simple because in addition to providing AC coupling capabilities, this application area usually requires the input signal to be DC coupled. Designing an active front end that provides good impulse response and low distortion performance (DC frequency ≥500MHz) is challenging. This article provides several design ideas and recommendations for analog front-ends used in high-performance ADCs suitable for high-speed data acquisition.

Figure 1: LMH6703 frequency response.

Using a differential amplifier is the preferred method of 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 selecting this type of device: gain-bandwidth product and the ability to set the op amp's common-mode output voltage from an external voltage. This is because it is important that the signal amplifier driving the ADC input sets the common-mode output voltage (VCMO) within the optimal ADC range. If these conditions are not met, the ADC's performance will degrade significantly as the disparity between the amplifier's VCMO and the ADC's optimal input common-mode voltage increases.

Figure 2: Secondary amplifier circuit diagram.

The main disadvantage of wideband differential op amps is that their gain is usually limited and their gain levels 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 equates to requiring an oversampling system with an input bandwidth of up to 500MHz.

For an op amp operating with a large gain (such as A _V =10) and maintaining such a large bandwidth, this 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 better relationship among these parameters because their performance is usually determined by the value of the feedback resistor within the op amp circuit. The LMH6703 operational amplifier is ideally suited for operation at high bandwidths with gain settings of 1 to 10. This 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 (RF1) of 300 ohms is obtained from Figure 1.

Therefore RG1 (gain resistor) can be selected as 33 ohms. Figure 2 is an example of a circuit using the LMH6703 with a differential amplifier.

In addition to requiring a fixed gain level system with a suitable DC signal path, this application also requires an AC coupled mode. This is because the DC signal path is typically limited by the gain bandwidth produced by the input amplifier. For data acquisition devices or communication channels that require wide input bandwidth and low distortion, we need to use AC signal channels. This extends the input frequency limit beyond the DC signal channel capacity.

There are many solutions, and the method chosen depends largely on the minimum input frequency and the required high-frequency performance. For the highest AC performance at high frequencies (≥200MHz), baluns provide a solution for single-ended to differential conversion because little added signal distortion is required. The trade-off is that baluns are lossy devices that attenuate the signal slightly (-1~2dB), and their low-frequency performance is poor. A balanced/unbalanced coupled signal path can be inserted into the circuit shown in Figure 3 by using a single-pole RF relay to switch the single-ended output signal from the preamplifier to a differential amplifier or balanced/unbalanced conversion circuit. Another SPDT RF relay is also required to forward the output of the balun transformer and differential amplifier into the ADC input.

Figure 4: FFT plot of a 198 MHz sine wave sent by a high-speed differential output op amp and sampled by the ADC08D500 at 500 MSPS.

This circuit is well suited for high-end test and measurement equipment. However, for cost-sensitive applications, the cost of RF signal relays creates a burden on the system budget, especially if multiple channels are required. Therefore, it would be advantageous for low-speed systems to select differential output op amps that can be used in both AC-coupled and DC-coupled modes, thus eliminating the need for balanced/unbalanced conversion circuitry. Amplifiers specifically suited to the task began to appear, gradually improving performance in terms of bandwidth and THD.

For an 8-bit 1GSPS converter, a differential amplifier with a minimum bandwidth of 1GHz that can provide -50dB THD at 500MHz is suitable. Better dynamic performance can be obtained from high-speed ADCs by leveraging off-the-shelf op amp components that can significantly reduce front-end design time. 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 by an 8-bit ADC at 500MSPS. The graph shows that the amplifier has very low 2nd and 3rd order harmonic distortion at this frequency, allowing the signal captured by the ADC to have noise and distortion values ​​comparable to the performance obtained from a dedicated AC coupled signal path.

Summary of this article

Amplifier performance continues to be improved to increase bandwidth and reduce THD. As ADCs enter the GSPS range, we need amplifiers that can interface with these converters. Eliminating circuit channels not only reduces system cost without sacrificing system performance, allowing designers to achieve higher performance at lower cost while reducing front-end circuit design time.