ELECTRONICS A Passive Current Source for Two-Wire Microphones Application Note 16 R1 1 0V VBat 4 53 k R2 1 3V 15 4 k 20 µA To Microphone Cc C To A/D Figure 1. Recommended passive network for supplying two-wire microphone with 20 µA current. Many Knowles microphones require three terminal connections, one each for supply voltage and ground, and a third for the output signal from the microphone. A Knowles two-wire microphone has only two terminal connections, where the supply and signal are on the same pin. The microphone is powered by a current source, and the output signal is separated from the current source through a coupling capacitor (Cc). satisfactory performance for many applications can be obtained with passive components using the circuit in Figure 1. The Knowles two-wire microphone requires 20 µA (nominal), and the recommended minimum compliance of the current source driving it is 0.9 V to insure maximum peak-to-peak output voltage. Power Supply Rejection (PSR) is a factor since the passive current source is connected directly to the hearing instrument battery. Figure 2 shows the PSR for various values of filter capacitor (C). The maximum capacitance values for a given form factor and For optimum performance, an active current source (i.e., composed of matched bipolar or MOS transistors) should be used. However, 90 C C C C 80 PSR (dB) 70 60 = 1µF = 10µF = 22µF = 47µF 50 40 30 20 10 0 2 3 4 5 6 7 2 8 910 2 3 4 5 6 7 3 8 910 2 3 4 5 6 7 8 910 4 Frequency (Hz) Figure 2. Power Supply Rejection of the current source with battery voltage of 1 volt and various values of the filter capacitor (C). ! Issue 060630 ! AN16 Two-Wire Circuit dielectric type are in Table 1. As an example, for a PSR of 40 dB at 200 Hz, choose a 22 µF capacitor; this value is available in a 0805 for ceramic or a 0603 for tantalum. The graphs in Figure 2 were generated from simulations. It is recommended that prototype circuits be evaluated prior to incorporation into a hearing instrument. Knowles Electronics can provide assistance if desired in the design and component selection for a particular application. Capacitor Size 0402 0603 0805 1206 Ceramic Tantalum 1 µF 10 µF 22 µF 22 µF not available 22 µF not available 47 µF Table 1. Maximum capacitor values for different package sizes and dielectric types. Knowles Electronics, LLC 1151 Maplewood Drive Itasca, Illinois 61034 USA +1 630 250 5100 voice +1 630 250 0575 fax www.knowleselectronics.com Knowles Europe York Road,Burgess Hill West Sussex, RH15 9TT England +44 1444 235432 voice +44 1444 248724 fax Knowles Electronics, Suzhou #179 SuHong Road Suzhou Industrial Park Suzhou, JiangSu 215021 China +86 512 6258 8258 voice +86 512 6258 9258 Fax Knowles Electronics (M) Sdn. Bhd. Plot 104 Lebuhraya Kampung Jawa Georgetown, Penang 11900 Maylasia +60 4 6437 466 voice +60 4 6437 446 Fax Knowles Electronics, Japan 5/F YK Building 2-16 Sangenjaya 2-Chome Setagaya-Ky, Tokyo 154-0024 Japan +81 3 3439 1151 voice +81 3 3439 8822 fax Micromax Pty Ltd 5 Orangegrove Avenue Unanderra NSW 2526 Australia +61 2 4271 1300 voice +61 2 4271 8091 fax www.micromax.com.au Note: All data and performance specifications in this document are for reference only. Page 2 of 2! Knowles Electronics