I. INTRODUCTION

Ultrasound scanners have found widespread utility across diverse medical applications due to their compact form factor and application-specific functionalities ^[1-5]. This paper introduces an analog front-end (AFE) amplifier for ultrasound application-specific-integrated-circuits (ASICs). The proposed AFE amplifier consists of a preamplifier with a single passband gain and a programmable gain amplifier (PGA) for a time-gain compensation (TGC).

Ultrasound scanning systems rely on TGC as a fundamental operation ^[6]. Ultrasound signals experience exponential attenuation in a medium, determined by factors such as a propagation distance and an acoustic signal frequency, as shown in Fig. 1(a). To ensure an accurate reconstruction of the instantaneous ultrasound scattering characteristics, it becomes imperative to compensate for the propagation-dependent attenuation ^[6].

There are relevant prior works to perform TGC operations in ultrasound ASICs ^[7-10]. While a continuous TGC operation closely approximates an ideal scenario ^[7,8], existing approaches have exhibited drawbacks such as intricate circuit topologies or the need for analog buffers due to a high output impedance.

Considering a complexity of circuit topology and a driving ability of TGC amplifier, a capacitive-feedback PGA can be an appropriate topology ^[9-11]. A voltage gain of PGA needs to be adjusted in a real time to accomplish a TGC operation along a scanning depth as shown in Fig. 1(b). One of primary concerns of real-time TGC operation is to suppress switching artifacts [2, 7, 10] during a gain transition. The primary objective of this work is to attain a seamless gain transition during a real-time TGC operation by using an operating point initialization method.

In this paper, we propose a preset operation for initializing the operating points of AFE amplifier. This preset operation effectively aligns the operating points of switching nodes, ensuring a seamless discrete TGC. In addition, to further enhance a transient response of AFE amplifier, this work proposes a current-reuse operational amplifier (op amp) employing dynamic push-pull current sources. The proposed op amps are deployed in each stage of AFE amplifier.

This paper is organized as follows. Section II addresses the issue of gain transition in the discrete TGC and presents a concept of proposed preset operation. Section III presents the circuit implementations of proposed AFE amplifier. Section IV shows the measurement results, and Section V concludes this work.

Fig. 1. Dynamic range of AFE amplifier output: (a) without TGC; (b) with TGC operation.

II. CONCEPT OF SEAMLESS DISCRETE TGC

Fig. 2 illustrates prior works of discrete TGC with PGA ^[9,10]. Both PGAs of Fig. 2 incorporate a reconfigurable series capacitor array to facilitate the adjustment of voltage gain through the manipulation of switches within the array. However, it is worth noting that these methods can introduce a common-mode fluctuation in the PGA output due to voltage discrepancies at critical nodes within the series capacitors.

In case of PGA of Fig. 2(a) ^[10], a left node of series capacitor is basically connected to a common-mode node of V$_{\mathrm{CM}}$. Meanwhile, the operating point of preamplifier output is typically determined by its feedback configuration, resulting in a slight difference between the preamplifier output and the voltage of V$_{\mathrm{CM}}$. During the gain switching moment, the left node of series capacitor is switched from the V$_{\mathrm{CM}}$ to the preamplifier output. The discrepancy in operating point between the preamplifier output and the level of V$_{\mathrm{CM}}$ leads to an input offset of PGA. To mitigate the fluctuation of output common-mode level of PGA, the work of ^[10] employs an analog buffer. The analog buffer temporarily shortens the relaxation time of feedback configuration, facilitating the rapid recovery of the targeted operating point. Measurement results in ^[10] demonstrate a transient time of less than 100 ns during gain transitions. Considering a typical center frequency of medical ultrasound, a further reduction of transient interval is preferable.

The PGA of Fig. 2(b) adjusts the voltage gain by switching the right node of series capacitor ^[9]. During the reset phase, the right nodes of series capacitors are connected to the node of V$_{\mathrm{CM}}$, and the summing node of op amp is set to its operating point, which is slightly different from the level of V$_{\mathrm{CM}}$. During the gain switching moment, the right node of series capacitor is switched from the node of V$_{\mathrm{CM}}$ to the summing node of op amp. The discrepancy of operating point of right node of series capacitor results in an equivalent input referred offset in PGA, generating a deviation of common-mode level in PGA output.

Fig. 3 shows the proposed preset operation to alleviate the voltage discrepancy of gain-switching node. A single-ended circuit topology is illustrated for simplicity. Similar to the work of ^[9], the voltage gain of PGA is adjusted by switching the right node of series capacitor. To initialize the right node of series capacitor and the summing node of op amp with the same operating point, the right node of series capacitor is also preset to the operating point of op amp during the reset phase. The voltage deviation attributable to charge injection of switches can be negligible as a result of the fully differential topology and sufficient large capacitance of series capacitors compared to a relatively small size of switch. After the reset phase, series capacitors are disconnected from the summing node of op amp. During the gain switching moment, the right node of the corresponding series capacitor is re-connected to the summing node of op amp. Since the time duration between the preset operation and the gain switching moment is less than 40 ${\mu}$s considering the maximum scanning depth, the charge loss due to the leakage current on the floated right node of series capacitor is negligible. Furthermore, this preset operation achieves the seamless gain transition with a relatively simple reconfigurable series capacitor array.

Fig. 2. Discrete TGC in prior works: (a) variable feedback-resistance based PGA; (b) summing-node-switching based PGA.

Fig. 3. Proposed preset operation for seamless discrete TGC: (a) circuit diagram; (b) timing diagram.

III. CIRCUIT IMPLEMENTATION

1. Overall Circuit Diagram of AFE Amplifier

Fig. 4(a) shows an overall circuit diagram of AFE amplifier. The AFE amplifier consists of a preamplifier and a PGA. The gain of both stages is determined by a capacitance ratio of series capacitor and feedback capacitor. The preamplifier maintains a fixed gain of 20 dB, while the PGA offers four selectable gain values from 16 dB to 33 dB. The PGA supports the proposed preset operation with the dedicated timing diagram.

The proposed AFE has two operational phases: a reset phase and an RX phase, denoted in Fig. 4(b). The reset phase corresponds to a typical ultrasound firing phase, so called as a TX phase, initializing each of nodes in the amplifier for the following echo amplification operation ^[6]. The proposed preset operation is concurrently executed during the reset phase, configuring the operating points of both nodes of series capacitors in PGA as the preamplifier output and the PGA output. After the reset phase, the gain-control switches of gain[2:0] are disconnected to make the gain of PGA the minimum value. As a scanning depth increases, each of gain-control switches is sequentially connected to the summing node of op amp with the pre-determined time interval ${\Delta}$t to perform the periodic discrete TGC operations.

Fig. 4. Proposed AFE amplifier: (a) circuit diagram; (b) timing diagram.

2. Op Amp with Dynamic Push-pull Current Sources

Each stage of AFE amplifier incorporates a fully differential op amp, as shown in Fig. 4(a). In this work, the employed op amp is based on a current-reuse topology ^[11-13]. Fig. 5(a) shows a schematic of a conventional current-reuse op amp. This amplifier reuses a constant bias current of I$_{\mathrm{Q}}$ for both NMOS and PMOS input differential pairs, leading to an increment of transconductance, a reduction of noise power, and an enhancement of power efficiency ^[11-13].

This work proposes a current-reuse op amp with dynamic push-pull current sources, as shown in Fig. 5(b). To illustrate the transient operation for instant increment and decrement of inputs of op amp, the increment or decrement of voltage and current are denoted as red color in Fig. 5(b). The push-pull current source corresponds to the flipped voltage follower (FVF), which is based on a feedback loop ^[14]. The drain-node voltage of input transistor in FVF operates oppositely to the changes in input voltage of FVF, contributing to an instant increase or decrease in pull-up or pull-down currents during the transient period. In case of Fig. 5(b), the input V$_{\mathrm{INP}}$ is increased and the input V$_{\mathrm{INN}}$ is decreased instantly during the transient period. Then, the pull-up current of M1 and the pull-down current of M4 instantly increase as a result of feedback loop in each FVF. Subsequent to this transient interval, the dynamic current sources maintain a quiescent current I$_{\mathrm{Q}}$. In a comparative analysis, the proposed op amp with dynamic current sources shows a 40% reduction in settling time, when both op amps with the same quiescent current are subjected to an abrupt input change in SPICE simulations. Hence, the proposed op amp facilitates a seamless discrete TGC operation by further reducing a transient interval.

Fig. 5. Current-reuse op amp with (a) constant tail current sources; (b) dynamic push-pull current sources.

IV. MEASUREMENT RESULTS

The proposed AFE amplifier has been implemented in a 180-nm BCD process with an active area of 0.19 mm$^{2}$ including a bias circuit, as shown in Fig. 6. Operating at a 1.8-V supply voltage, a total power consumption of AFE is 2.3 mW. Considering several primary specifications such as the center frequency of 4 MHz, the maximum scanning depth of 4 cm, and the attenuation coefficient of 0.7 dB/(MHz${\cdot}$cm), the required TGC amount for the maximum scanning depth corresponds to 22.4 dB. We set a number of discrete TGC moments as three, as illustrated in Fig. 1. Correspondingly, we determined the specifications of PGA to cover the gain range of 16.8 dB with four gain steps.

Fig. 7 shows the measured frequency response of AFE. It shows four discrete gain steps from 35.7 dB to 53 dB at a low frequency band. The bandwidth of AFE is 4.5 MHz at the maximum gain setting, effectively supporting the bandwidth of the target transducer. At the target center frequency of 4 MHz, the gain of PGA spans from 34.3 dB to 50.5 dB, featuring an average gain step of 5.4~dB.

Fig. 8 demonstrates the real-time TGC operations in the measured time-domain waveforms. We applied a sinusoid signal with a frequency of 4 MHz as the input for the implemented AFE amplifier, as shown in Fig. 8(a). During each of gain switching moments, the output of AFE amplifier was further amplified, showing a seamless gain transition with a transient interval less than 18 ns. In Fig. 8(a), the measured DC-level shift is less than 7 mV, which corresponds to the input-referred offset of 21 ${\mu}$V. Considering the typical ultrasound signal processing such as a band-pass filtering and an envelope detection, it is estimated that this DC-level shift rarely causes the switching artifact in an ultrasound image. Due to the limitation of measurement setup, we applied the emulated echo signals as the input (Fig. 8(b)), rather than real ultrasonic echoes. The output waveforms show the consistent peak-to-peak amplitude, demonstrating the uniform gain steps in the real-time TGC operations.

Fig. 9 shows the simulated input-referred voltage noise density of the implemented AFE amplifier. The input referred noise density at the center frequency of ultrasound is 6.3 nV/${\sqrt{}}$Hz. As a result of utilization of the current-reuse op amp and an optimization of transconductance of transistors in op amp ^[12], it shows a sufficient noise performance as the AFE amplifier.

Table 1 compares the proposed AFE amplifier with the prior works of ^[7-11]. The proposed AFE amplifier performs a discrete TGC operation for the voltage input signal. The real-time discrete TGC operation of proposed AFE amplifier was verified, demonstrating a transient interval less than 18 ns. Considering a typical center frequency of medical ultrasound applications, it is a sufficiently small value, estimated not to generate the imaging artifacts associated with the discrete TGC operation.

Fig. 6. Chip micrograph and layout.

Fig. 7. Measured frequency response of AFE amplifier.

Fig. 8. TGC operations in measurements: (a) a case of applying a single-frequency sinusoid signal; (b) a case of applying exponentially attenuated echo signals.

Fig. 9. Simulated the input-referred voltage noise density of AFE amplifier.

V. CONCLUSIONS

This paper presents an AFE amplifier with a seamless discrete TGC operation for an ultrasound scanner. The proposed AFE amplifier consists of a preamplifier and a PGA. We analyze an issue of fluctuations of operating points of amplifier during a gain switching moment of discrete TGC operation. To mitigate this issue, we propose a preset operation during a reset phase, minimizing a discrepancy of voltage levels in critical floating nodes. Additionally, we propose a current-reuse op amp with dynamic push-pull current sources. We employ an FVF to replace the constant tail current sources, further improving a transient response for a discrete TGC operation. The proposed AFE amplifier has been implemented in a 180-nm BCD process, occupying an active area of 0.19 mm$^{2}$ with a power consumption of 2.3 mW. In measurements, a gain range of AFE amplifier is 16.2 dB with a gain step of 5.4 dB. A real-time discrete TGC operation is also demonstrated, showing a transient interval less than 18 ns at a gain switching moment. Considering a target center frequency of ultrasound, the measured transient interval value is estimated not to result in the imaging artifacts associated with the gain switching.