Reference Voltage for Multiple

文章摘要：Abstract: This paper discusses the successful implementation of two reference circuits tailored to the space-saving needs of ultrasound imaging applications, which commonly encounter the demand for one reference source to power multiple analog-to-digital converters (ADCs). Finally, the two circuits

Reference Voltage for Multiple,标签：模拟电子技术基础,模拟电子电路,http://www.88dzw.com
Abstract: This paper discusses the successful implementation of two reference circuits tailored to the space-saving needs of ultrasound imaging applications, which commonly encounter the demand for one reference source to power multiple analog-to-digital converters (ADCs). Finally, the two circuits were put to the test and their results are discussed in the following note.

The achievable accuracy for systems with multiple analog-to-digital converters (ADCs) depends directly on the reference voltage(s) applied to the ADCs. Medical ultrasound imaging systems, for example, commonly include a large number of ADCs in the receiver's beamformer electronics, usually organized in groups of 16, 24, 32, etc. Maximum beam accuracy requires that you minimize errors in this path. Poor accuracy in the reference voltage of individual ADCs degrades the overall system, as does the distributed load, which consists of many individual resistive and capacitive loads. Several approaches can provide the reference voltage for such ADC arrays:

• Individual on-chip references: Though it offers a convenient connection locally to each ADC, this option features relatively poor matching between the converters.
• A single external reference voltage applied to all reference inputs of the ADC array: Such a configuration lets you engineer an external reference voltage of arbitrary accuracy, but incurs error due to small variations among the resistor ladders (one ladder internal to each ADC).
• An external reference driving the ADCs' reference ladder taps directly: This option delivers maximum gain accuracy by directly controlling the reference voltage applied to each ADC ladder. However, it requires driving the (relatively) low resistance of the ladders, and some ADCs do not allow access to that internal bias point.

ADC Accuracy

For many applications, gain and noise level have a major effect on ADC accuracy. The gain of an ADC is represented by the slope of its transfer function, which relates analog inputs to the allowable range of digital output codes. One way to quantify gain is to measure the full-scale (FS) input range, which is directly controlled by the reference voltage level. For medical ultrasound imaging systems, variation in the full-scale range of the ADCs can cause errors in beam formation. This also varies the ADCs' clipping point—an effect that may be important in certain signal demodulation schemes.

An ADC's noise level determines its useable dynamic range; this dynamic range should generally be as large as possible. The reference noise component of ADC noise can be additive or multiplicative. Additive noise is easily filtered by local bypass capacitors on the individual ADCs, which in most designs are already present to optimize the ADC's dynamic performance.

Multiplicative noise, on the other hand, is more insidious. For ultrasound applications, reference noise in the audio frequency spectrum can modulate large "stationary" signals in the RF spectrum. Such signals are produced by stationary tissue in the ultrasound target. Audio modulation produces sidebands on the RF signal that can be demodulated by a Doppler detector, producing audio tones in the detected Doppler output signal.

To estimate the amount of audio noise tolerable in an ultrasound application, assume a nearly full-scale RF signal applied to a 10-bit ADC like the MAX1448. The dynamic range of that device (almost 60dB) implies a noise floor of -60dBFS. That noise level can be normalized to a 1Hz bandwidth. The Nyquist bandwidth for 80MHz sampling rate would be 40MHz. The correction factor is √(40MHz)= 76dB, which places the ADC's noise floor at -60dBFS - 76dBFS = -136dBFS. Because a conservative design requires the reference voltage noise to be at least 20dB lower (-156dBFS), a +2.0V reference requires an extremely low noise level of 33nV

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Tag:模拟电子技术，模拟电子技术基础,模拟电子电路，模拟电子技术