Volume 41 – September 2007
Toward More-Compact Digital Microphones
For many years, microphones used in telecom applications have been of the electret condenser (ECM) type. The microphone comprises a membrane, a back plate, and an electret layer. The movable membrane and fixed back plate are the plates of a variable capacitor. The electret layer stores a fixed charge corresponding to a capacitor voltage of approximately 100 V. Sound pressure will cause the membrane to move, varying the capacitance of the microphone. Since charge on the capacitor is constant, the voltage across the capacitor will vary with the changing capacitance, based on the formula for charge on a capacitor:
Q is the charge, in coulombs, C is the capacitance, in farads, and V is the voltage, in volts. The minuscule increases and decreases in capacitance, ΔC, with sound pressure, cause proportional decreases and increases in voltage, ΔV.
Microphones for mobile applications are quite small, typically 3 mm to 4 mm in diameter and 1 mm to 1.5 mm in thickness. Consequently, their capacitance is also relatively small. Typical values are of the order of 3 pF to 5 pF, and in some cases, as little as 1 pF.
Having no drive strength, the signal produced by a capacitive microphone needs a buffer/amplifier prior to further processing. Conventionally this microphone preamplifier has been implemented using a simple junction field-effect transistor (JFET). Figure 1 shows a cross-section of a packaged JFET-based ECM.
Figure 1. JFET-based microphone cross-section.
As micromachining of electret microphones has improved, microphones have become smaller, and their element capacitance has decreased. Standard JFETs no longer suffice because their relatively large input capacitance significantly attenuates the signal from the microphone cartridge element. Fortunately, improvements in CMOS process technologies have led to improvements in amplifier circuits. Much is gained by replacing JFET-based amplifiers with CMOS analog and digital circuitry. Preamplifiers implemented in modern submicron CMOS processes have enabled, and will further enable, a wide range of improvements over traditional JFETs:
Digital-Output Microphone Preamplifier
Mobile phones present an inherently noisy environment. A drawback of the traditional JFET (and indeed any purely analog) solution is that analog microphone output signals can easily be corrupted by interfering signals creeping in between the amplifier and the analog-to-digital converter. Thus, incorporating analog-to-digital conversion into the microphone itself provides a digital output that is inherently less prone to corruption by interferers.
Figure 2. Digital microphone system diagram with the ADAU1301 microphone preamplifier.
The preamplifier is built in CMOS using two operational transconductance amplifiers (OTAs) in an instrumentation-amplifier configuration where the gain is set using matched capacitors. This configuration, with its MOS input transistors, presents a highly desirable near-zero input admittance to the capacitive signal source. The use of capacitors for gain setting allows high gain accuracy—limited only by process lithography—and the inherently high linearity of poly-poly capacitors. The gain of the amplifier is easily set by metal-mask programming, allowing gains of up to 20 dB. The analog-to-digital converter is a fourth-order, single-loop, single-bit Σ-Δ modulator, whose digital output is a single-bit oversampled signal. Using a Σ-Δ modulator for analog-to-digital conversion offers several advantages:
A potential problem with higher-order Σ-Δ modulators is that they are prone to instability when the input exceeds the maximum stable amplitude (MSA). Higher-order modulators (>2) fail to return to stable operation when they become unstable due to overload, even when the input is reduced below the MSA. To counter potential instability, a digitally controlled feedback system alters the Σ-Δ noise transfer function, forcing the modulator back into stable operation.
A power-down mode, entered by allowing the system input clock frequency to drop below 1 kHz, lowers the current drawn by the system from 400 μA to approximately 50 μA, allowing the user to conserve power whenever the microphone is not needed. The start-up time from power-down is only 10 ms.
As a failure analysis feature, a special test mode enables access to various internal nodes in the circuit. A special preamble at the DATA pin during startup allows the failure analysis engineer access by switching these nodes to the DATA pin.
Flicker-noise spectral density has an inverse dependency on transistor area; its magnitude, referred to the input, is given by
where Kf is a process-dependent constant, f is frequency, W is the MOS width, L its length and Cox is the gate capacitance per unit area. The 1/f-noise amplitude can be reduced by increasing the size of the input transistors. The input-referred white noise is inversely proportional to the transconductance, gm, of the metal-oxide-semiconductor transistor (MOST)
where k is Boltzmann’s constant and T is absolute temperature. For a MOST in strong inversion, gm≈ 2Id/Veff, where Id is the drain current, and the effective voltage, Veff = Vgs – Vth, the gate-to-source voltage minus the MOST threshold voltage, Vth. By designing the input pair to be very wide, a bipolar-like mode of operation is imposed upon the MOST as it enters the weak inversion operating mode. Here, gm = Id/(nVT), where n is the slope factor (typically 1.5) and VT is the thermal voltage. Thus, optimum white noise performance is achieved by maximizing the MOST aspect ratio.
The input bias resistor is connected to a capacitive source, so its noise will be low-pass filtered. Assuming that the noise is low-pass filtered white noise and the cutoff frequency is much smaller than the audio-band frequencies, it can be shown that the total noise power is kT/C, where C is the capacitance connected to the node.
As a consequence of the trend toward smaller microphone cartridges with lower cartridge capacitance, this noise source will increase as the microphone cartridge capacitance decreases. However, the audio-band noise power generated by the bias resistor will also depend on the cutoff frequency of the low-pass filter. The lower the cutoff frequency, the smaller the amount of the total noise power remaining in the audio frequency range. In order to keep the noise low, the value of the bias resistance will have to be increased by a factor of four for each halving of the microphone capacitance. For a 3-pF to 5-pF microphone capacitor, the resistor should have a minimum value of approximately 10 gigaohm.
A good solution for implementing such large value resistors on chip is a pair of antiparallel diodes which have a very large resistance around equilibrium, typically 1 teraohm to 10 teraohms. The resistance decreases for larger signals, assuring fast settling after overload situations. Figure 3 shows the in-band noise as a function of RBIAS.
Figure 3. Noise from bias resistor.
The area of the input transistors of the preamplifier must be optimized in relation to the microphone capacitance. Although, as noted earlier, the 1/f noise will decrease if the input devices are made very large, the capacitive loading of the signal source will increase, attenuating the signal and reducing the wideband signal-to-noise ratio (SNR). This presents a trade-off: If the input device is made very small, the capacitive loading of the signal source becomes insignificant, but the 1/f noise increases dramatically, reducing low-frequency SNR. The optimum for maximizing SNR with respect to 1/f noise exists where the gate-source capacitance of the input device equals the microphone capacitance plus parasitic capacitance. The optimum for white noise exists where the gate-source capacitance of the input device equals one-third of the microphone capacitance plus parasitics. In practice, the best compromise is for the gate capacitance to fall between the two values.
Bootstrapping minimizes the input pad contribution to the overall chip input capacitance. As the output-referred white noise is proportional to gm, all current-source MOSTs are biased in the strong inversion region, ensuring minimum noise contribution.
Table 1 shows the key characteristics and performance of the ADAU1301 microphone preamplifier.
Table 1. Typical (Unless Noted) Characteristics and Performance of the ADAU1301
Toward a Fully Integrated Digital Microphone
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