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A novel peak-statistical algorithm and judgment logic (PSJ) for multifrequency signal application of Autogain Control Loop (AGC) in hearing aid SoC is proposed in this paper. Under a condition of multifrequency signal, it tracks the amplitude change and makes statistical data of them. Finally, the judgment is decided and the circuit gain is controlled precisely. The AGC circuit is implemented with 0.13

In the twenty-first century, as China entered the aging society gradually, hearing damage has become a major disease that is prevalent in elderly people. In this group, more than 90% of patients can compensate and repair their hearing ability by wearing a hearing aid device. A digital hearing aid SoC includes Autogain Control Loop (AGC), ADC, digital signal processing platform (DSP), power amplifier driver, and EEPROM [

As the most significant part of SoC, the performance and power consumption of AGC are decisive for the SoC. Traditional AGCs use analog peak envelop-detection method which limits the resolution and dynamic range of gain adjustment. It also consumes extra power and worsens the noise performance [

According to the characteristics of audio signal, a novel peak-statistical algorithm and judgment logic (PSJ) for AGC are proposed in this paper. It extracts the amplitude statistical characteristics of most signal and adjusts AGC gain precisely, which ensure that most signal remains in the best receiving range. The algorithm is realized by mixed-signal design method. As a logic circuit, the PSJ circuit consumes less power compared with its analog opponent. Also by adopting low-power topology, the power consumption of circuit is optimized and high performance of SNR and THD is achieved.

Previous research proves that the sound pressure level (SPL) received by human beings ranges from 60 dB SPL to 100 dB SPL, and the safe sound level is about 80 dB SPL to 90 dB SPL [

Block diagram of AGC circuit.

Firstly, the microphone signal is acquired and amplified (or attenuated) by PGA. Then, the PGA output is compared with peak-threshold (

The motivation of the new algorithm is as follows: Taking single-frequency input signal for example,

Since AGC’s sampling clock frequency is much higher than the sound signal frequency, even if the input (

Basis of PSJ logic with single-frequency input. (a) When

In a more complex multifrequency signal environment as Figure

Basis of PSJ logic with multifrequency input.

The statistical properties of the output signal are derived as follows: when the AGC input is the sine wave, the probability of PSJ is calculated as

And for a given voltage, the probability density of a sine wave is

Then, the signal probability between

Based on

When voltage supply is 1 V and the full input swing of Sigma-Delta modulator is 400 mV, in order to avoid output distortion, the optimized modulator input is about 250 mV. So

Through simulation, to get precise signal information, it needs to be sampled 600 times for a gain adjustment cycle. On the basis of the probability calculation, we have to get at least 31 sampling points characteristics to judge the right amplitude range of input signal. It means that when more than 31 sampling points output logic “11,” the signal amplitude is considered greater than

Under low voltage supply such as 1 V, the OTA design is always a great challenge [

OTA circuit.

Also, the input PMOS transistors can minimize the equal input noise due to its low flick noise, which means for the same output amplitude level the SNR is improved [

In this paper, a 2nd-stage CMFB amplifier is adopted for OTA design. In this OTA, the first and second pole are in the drain of PM2/NM2 and PMC2/NMC2, respectively. And they are also very close. Since the third pole is located in the drain of PM5/NM5, the zero-resistance R1/R2 is significant for offsetting the third pole influence to ensure enough phase margins. Finally, the designed OTA demonstrates 83-dB DC gain, 29-MHz unity gain bandwidth, and 61-degree phase margin for a 2-pF load.

For high-resolution comparator design, the dynamic comparator with preamplifier is used in this paper [

Comparator circuit.

Fabricated with SMIC 0.13 ^{2}.

The chip microphotograph of AGC.

When input 1 kHz, 200 mVpp sine wave and sampling clock is 1 MHz, the output of AGC is tested in time domain. As Figure

AGC output in time domain.

Figure

FFT result of output spectrum.

As the important performance of AGC, the noise performance is also measured. We break the AGC close loop and set the PGA gain manually. Under 2 kHz frequency and 4 mV Vp-p input, Figure

The relationship between PGA gain and noise floor.

The result shows that the minimum and maximum noise floor are −111 dBm and −73 dBm, respectively, which are low enough to maintain a high resolution of AGC.

Figure

Measured SNR versus input.

The proposed AGC performance summary is concluded in Table

AGC performance summary.

Parameter | AGC |

Process | SMIC 0.13 |

Supply voltage | 1 V |

Signal width | 8 kHZ |

Minimum noise floor | −111 dBm |

SNR peak | 69.2 dB@200 mVp-p |

THD peak | 65.3 dB@200 mVp-p |

Power consumption | 89 |

Area | 1.15 × 0.98 mm^{2} |

The performance comparison of our AGC with previous works is shown in Table

Performance comparison of AGC.

Ref | Technology | Supply voltage (V) | Peak THD (dB) | Power consumption (uW) |
---|---|---|---|---|

[ | 1.5 um | 2.8 | 40 | 34 |

[ | 0.25 um | 1 | 44.4 | 60 |

[ | 0.35 um | 1 | 41 | 100 |

[ | 0.13 um | 0.8 | 64 | 40 |

This work | 0.13 um | 1 | 65.3 | 89 |

In this paper, a novel PSJ feedback logic for multifrequency signal application of AGC is proposed. In complex audio signal condition, compared with traditional analog peak envelop-detecting method, it can adjust the gain precisely with 3 dB/step, total 36 dB dynamic range. The PSJ algorithm is implemented in an AGC with 0.13 ^{2} core area. Measurement results satisfy the low-power and high-performance application of hearing aid SoC.

The authors declare that they have no competing interests.

This work was supported by the National Natural Science Foundation of China (no. 61674087) and the National Natural Science Foundation of China (no. 61674092).