Ultrasonic receiver VGA output reference noise and gain tuning

Phased array receiver overview

We analyze key VGA (variable gain amplifier) parameter on the Doppler effect, first look at the typical ultrasonic phased array consisting of the receive channel of the basic unit and how it works, a typical receiver includes LNA, VGA, anti-aliasing filter and ADC (as shown in Figure 1), LNA zoom from the sensor to the single ended 15MHz 1MHz input signal, LNA gain with approximately 19dB and 50 Ω to 1 K Ω input impedance of a source, you can optimize the selection so that it matches with the sensor, and ultra low noise figure.

Immediately after receiving launched pulse cycle start phase, LNA input signal amplitude may reach 0.5Vp-p.

Throughout the process, the received signal strength will gradually decay, eventually dropping to receiver noise floor, a known human body acoustic energy attenuation rate about 0.7dB/cm-MHz (journey to 1.4dB/cm-MHz), sound waves in the body of the transmission speed for 1540m/s (journey to 13 μ s), so you can calculate the attenuation values. In the receive cycle processing the signal the dynamic range of about 110dB, far beyond the actual dynamic range of ADC converter, therefore, receive cycle using VGA (below referred to as "time gain control") dynamic increase receiver gain to the signal to be able to adapt to the ADC input dynamic range. If you want to receive signals can accommodate 12-bit ADC dynamic range 70dB, require VGA gain range is approximately 40dB. In Figure 1 receive link, three pole anti-aliasing filter prevents the ADC is the largest imaging 15MHz frequencies above high-frequency noise and other signal, usually 12-bit ADC, its operating frequency is between 40Msps-60Msps.

VGA output reference noise and gain and the influence of PW Doppler

Standard 2D, grayscale, ultrasound imaging requires each phased array channels of dynamic range of about 40dB.

However, from the blood received signal strength may be far below the signal strength of the surrounding tissue, pulse-Doppler imaging device, such as spectral PW Doppler imaging and color Doppler imaging requires dynamic range 70dB high reach, usually large dynamic range of 12-bit ADC, to improve the performance of Doppler receiver.

Design meets ultrasound receiver system VGA with ADC requires considerable difficulty, in particular: to maintain a low noise output reference to maintain receiver dynamic range while still able to provide enough gain to guarantee high TGC (time gain control) gain the following receiver with low noise figure, will be an elusive goal, the actual design of VGA, lower output reference noise and high maximum gain usually is contradictory, in this case, the designers had to VGA to VGA parameter optimization and a reasonable balance in order to ensure the best overall performance of the receiver.

In order to better understand these VGA parameters on receiver performance impact, we consider two special cases.

A scenario is moderate or gain TGC gain is relatively low, and accept the signal strong, in this case, you should focus on optimization in receiver's dynamic range, another scenario is when the largest TGC-gain, while receiving signal is low, which is most important that it should be on receiver noise system is optimized in order to maintain the receiver sensitivity.

VGA output reference noise on the influence of dynamic range receiver

In the medium/low TGC gain, VGA output noise mainly VGA output reference noise, the noise should be well below the noise floor, the ADC will degrade ADC dynamic range, as shown in Figure 1 ultrasonic receiver system, for example, VGA output MAX2037 reference noise about 22nV/, 12-bit, 50Msps MAX1473 ADC for digital VGA output, with the noise floor 31.7nV/.

Assuming that the ADC maximum input voltage, 70dB of 1.4Vp-p SNR. In this case, if the VGA and ADC between anti-aliasing filter with 0dB attenuation of the band, because the VGA output reference noise impact, 70dB ADC's SNR decreases 1.7dB effectively, reduce to 68.3dB. However, the vast majority are used to this kind of system anti-aliasing filter has the band attenuation, in order to ensure system stability, many VGA require some form of real output impedance for driver filters, these impedance must be large enough to ensure that the filter capacitance is not infinitely small, this limitation is usually given the actual introduction of anti-aliasing filter 3dB-6dB of band attenuation, ADC inputs, anti-aliasing filter band attenuation further reduces output reference noise and improves the dynamic range. If the band attenuation-6dB, MAX2037 output reference noise will only make the ADC's SNR decreases 0.49dB.

It is easy to understand, if VGA with much greater than the output reference noise MAX2037 will cause problems, for example, when using 6dB attenuation of anti-aliasing filter, the size of only 40nV/VGA output reference noise (value $ MAX2037 output reference noise in two times), will cause the ADC's SNR decreases 1.5dB, Doppler Imaging more difficult, the attenuation of the more important.

In addition, it is important to note that anti-aliasing filter attenuation caused by receiving gain, reduce noise figure for GPS receivers have significant negative effects, this is our detailed instructions below, MAX2037 output reference noise about half of the competitors, and 12-bit ADC and passive anti-aliasing filter in conjunction, MAX2037 offers higher gain to optimize the dynamic range, and to maintain the receiver noise figure, Figure 2 shows the output reference noise MAX2037 and gain correspondence between curves.

VGA maximum gain on receiver noise coefficient of (high gain TGC)

Gain high TGC, you need to be optimized for receivers to improve sensitivity small signal, VGA output reference noise of total and ADC's noise should be far less than the ADC input sensor noise is enlarged.

Figure 3 is a simplified diagram of ultrasonic receiver, the diagram shows the receiver before ADC gain on effect of noise coefficient, the receiver systems gain as 19dB of MAX2034 four channel LNA, maximum gain for the MAX2037 29.5dB VGA and eight channel 12-bit ADC MAX1437 while also assumes that anti-aliasing filter with 6dB of band attenuation, suppose sensor impedance is 200 Ω, produced by thermal noise floor, or 1.8nV Figure 4, suppose the LNA's Zin 200 Ω, LNA input terminals of thermal noise floor is approximately half of the value (0.9nV/), in a typical LNA, VGA and ADC noise indicators, in this case the receiver system noise figure of approximately 2.3dB. The noise floor for MAX1437 31.7nV/, when the gain maximum TGC, ADC before system (including anti-aliasing filter) gain as 42.5dB. At this point, the ADC is relative to the receiver input noise only 0.237nV/total 2.3dB receiver noise figure, ADC only 0.18dB.

The maximum gain VGA low noise floor of ADC or higher-what happens when? Figure 4 shows the VGA gain on Figure 3 shows a typical small ultrasonic receiver signal noise figure, assuming that the receiver system uses gain for low noise 19dB ultrasonic attenuation for LNA and 6dB of anti-aliasing filter, we were given two different noise floor of the ADC is the noise factor graphs, diagrams above a curve corresponds to MAX1437, MAX1473 maximum input voltage 1.4Vp-p, SNR-70dB, noise floor about 31.7nV/; following a curve corresponds to 2Vp-p input, ADC SNR-70dB, the ADC's noise floor about 45.2nV/.

The figure clearly indicates that the receiver noise coefficient on these two different ADC, shows MAX2037 as high as the maximum gain 29.5dB on receiver noise coefficient of improvement.

The maximum gain of VGA is low, will make you gain maximum TGC's overall noise coefficient increased receiver, while reducing small signal Doppler sensitivity.

Reasonable choice of low noise floor, for example MX1437 ADC, and a higher maximum gain, such as the VGA, can significantly improve MAX2037 noise figure.

Conclusion

Concerned with VGA output reference noise, maximum VGA gain, anti-aliasing filter attenuation and ADC dynamic range and noise on the receiver and the influence of noise coefficient is very necessary to help optimize ultrasound receiver sensitivity.

Optimization using MAX2037 VGA and reasonable balanced output reference noise and maximum gain to meet a 12 bit ADC (e.g. MAX1437) performance requirements, which are the best indicators of ultrasonic receiver.