SLAAE64 may   2023 AFE58JD48 , DAC81001 , DAC8801 , DAC8830 , OPA2210 , REF5010 , REF5040 , THS4130

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Proposed Topologies
    1. 2.1 Proposal 1: Using R-2R DAC (DAC8830)
      1. 2.1.1 Highlighted Products
        1. 2.1.1.1 DAC8830
        2. 2.1.1.2 OPA2210
        3. 2.1.1.3 THS4130
        4. 2.1.1.4 REF5040
      2. 2.1.2 Design Circuit
      3. 2.1.3 PSpice-TI Simulation
    2. 2.2 Proposal 2: Using M-DAC (DAC8801)
      1. 2.2.1 Highlighted Products
        1. 2.2.1.1 DAC8801
        2. 2.2.1.2 OPA2210
        3. 2.2.1.3 THS4130
        4. 2.2.1.4 REF5010
      2. 2.2.2 Design Circuit
      3. 2.2.3 PSpice-TI Simulation
    3. 2.3 Proposal 3: Using Low-Noise R-2R DAC (DAC81001)
      1. 2.3.1 Highlighted Products
        1. 2.3.1.1 DAC81001
        2. 2.3.1.2 OPA2210
        3. 2.3.1.3 THS4130
        4. 2.3.1.4 REF5010
      2. 2.3.2 Design Circuit
      3. 2.3.3 PSpice-TI Simulation
  6. 3Conclusion
  7. 4References

1 Introduction

Ultrasound imaging is based on the pulse-echo method, by sending ultrasound waves to the objects being imaged and receiving echo signals. It is also known that the emitted ultrasound wave amplitude gets smaller as the wave penetrates tissue, a phenomenon called attenuation. Signals that are reflected immediately after transmission are very strong because the signals are from reflections close to the surface; reflections that occur long after the transmit pulse are very weak because these signals are reflecting from deep in the body. If the ultrasound image was formed directly by the raw returned echoes, the image appears lighter in the superficial layers and darker in deep layers. A way to overcome ultrasound attenuation is time gain control or compensation (TGC), in which signal gain is increased as time passes from the emitted wave pulse. This correction makes equally echogenic tissues look the same even if the tissue is located in different depths. TI’s low-noise analog front ends (AFEs) have a TGC feature that supports ultrasound applications because the AFEs can alter the gain of the receiver as a function of time. The ultrasound signal incident on the receiver decreases in amplitude as a function of the time elapsed since transmission, and the TGC helps achieve the best possible signal-to-noise ratio (SNR), even with the decreasing signal amplitude.

This application note describes the specifications and design considerations of three proposal circuits used for generating a time-varying control voltage to drive multiple AFE receiver chips. Figure 1-1 shows the signal chain of the AFE58JD48 analog front end. The TGC function is integrated and is implemented using an attenuator that can be controlled with a control voltage, VCNTL. External circuitry comprised of a digital-to-analog converter (DAC) and operational amplifier (op amp) generates the control signal. The input signal for the DAC is a time-varying digital control from a field programmable gate array (FPGA) that can also handle the beamforming operation required in an ultrasound application.


GUID-20230131-SS0I-F25G-SQRB-WV2HWR36NN4M-low.svg

Figure 1-1 Signal Chain of the AFE58JD48 and Analog Control for TGC Operation

VCNTL attributes from the point of view of the AFE are provided in the following list.

  • Signal levels on the control pins of AFE: In the case of the AFE58JD48, VCNTL (= VCNTLP – VCNTLM) is a differential input for controlling the voltage attenuation, ranging from –0.4 V to 0.4 V. The common-mode voltage for VCNTLP, VCNTLM is 1.3 V typically. This control voltage varies the attenuation of the attenuator based on the linear-in-dB characteristic. For single-ended operation, VCNTLM can be fixed to 1.3 V and VCNTLP can be swept from 0.9 V to 1.7 V. For fully-differential operation, (VCNTLP, VCNTLM) goes from (1.1 V, 1.5 V) to (1.5 V, 1.1 V). Figure 1-2 shows the relationship between VCNTL voltage and VCAT attenuation. When the differential voltage level (VCNTLP – VCNTLM) exceeds the range of (–0.4 V to 0.4 V), the attenuator continues to operate at the maximum or minimum attenuation level.

    GUID-20221116-SS0I-TFM0-SFHT-PZ3NHXZVBS8C-low.svg

    Figure 1-2 Relationship Between VCNTL Voltage and Attenuation
  • Input referred noise: As the received ultrasound signal decreases as a function of elapsed time, VCNTL also decreases to reduce the attenuation and increase the channel gain. Figure 1-3 shows the benefit of the TGC circuit. As VCNTL increases and the channel gain increases, the input-referred noise of the receiver also continues to decrease. The reduced noise helps arrest the SNR fall related to the declining amplitude of the receiver signal.

    GUID-20221116-SS0I-RGQ2-BQH1-VMFTDC4LLSQ0-low.svg

    Figure 1-3 Input-Referred Noise vs VCNTL and LNA Gains for Low-Noise Mode
  • Noise requirement for multiple channels: One key consideration in the design of the VCNTL drive circuit is the noise specification on VCNTL. Since VCNTL is a common control voltage across multiple channels of the AFE (and possibly shared with the channels of other AFE chips), any noise on VCNTL shows up as a source of noise that is correlated across the multiple AFE channels that share that same VCNTL. Noise at the VCNTL pins must be low enough to obtain good system performance because this noise is correlated across channels. Figure 1-4 shows the allowed noise on VCNTL as a function of the number of channels sharing the same VCNTL drive.

    GUID-20221117-SS0I-RQWG-DH7R-0PQX8BSPKFHM-low.svg

    Figure 1-4 Allowed Noise on the VCNTL Signal Across Frequency and Different Channels