LMV1091 Dual Input, Far Field Noise Suppression Microphone Amplifier General Description Key Specifications The LMV1091 is a fully analog dual differential input, differential output, microphone array amplifier designed to reduce background acoustic noise, while delivering superb speech clarity in voice communication applications. The LMV1091 preserves near-field voice signals within 4cm of the microphones while rejecting far-field acoustic noise greater than 50cm from the microphones. Up to 20dB of farfield rejection is possible in a properly configured and using ±0.5dB matched micropohones. Part of the Powerwise™ family of energy efficient solutions, the LMV1091 consumes only 600μA of supply current providing superior performance over DSP solutions consuming greater than ten times the power. The dual microphone inputs and the processed signal output are differential to provide excellent noise immunity. The microphones are biased with an internal low-noise bias supply. ■ ■ ■ ■ ■ ■ ■ ■ Far Field Noise Suppression Electrical * SNRIE Supply voltage Supply current Standby current Signal-to-Noise Ratio (Voice band) Total Harmonic Distortion + Noise PSRR (217Hz) 34dB (typ) 26dB (typ) 2.7V to 5.5V 600μA (typ) 0.1μA (typ) 65dB (typ) 0.1% (typ) 99dB (typ) *FFNSE at f = 1kHz Features ■ ■ ■ ■ ■ ■ ■ ■ No loss of voice intelligibility Low power consumption Shutdown function No added processing delay Differential outputs Adjustable 12 - 54dB gain Excellent RF immunity Available in a 25–bump micro SMD package Applications ■ ■ ■ ■ Mobile headset Mobile and handheld two-way radios Bluetooth and other powered headsets Hand-held voice microphones System Diagram 30092240 © 2009 National Semiconductor Corporation 300922 www.national.com LMV1091 Dual Input, Far Field Noise Suppression Microphone Amplifier October 28, 2009 LMV1091 Typical Application 30092215 FIGURE 1. Typical Dual Microphone Far Field noise Cancelling Application www.national.com 2 LMV1091 Connection Diagrams 25ump micro SMD package 30092214 Top View Order Number LMV1091TM See NS Package Number TMD25AAA 25–Bump micro SMD Marking micro SMD Package View 30092216 Bottom View 30092231 Top View X = Plant Code YY = Date Code TT = Die Traceability ZA4 = LMV1091TM Ordering Information Order Number Package Package Drawing Number Device Marking LMV1091TM 25 Bump µSMD TMD25AAA ZA4 250 units on tape and reel LMV1091TMX 25 Bump µSMD TMD25AAA ZA4 3000 units on tape and reel 3 Transport Media www.national.com LMV1091 TABLE 1. Pin Name and Function Bump Number Pin Name Pin Function Pin Type A1 A2 MIC BIAS Microphone Bias Analog Output MIC2+ Microphone 2 positive input Analog Input A3 MIC2– Microphone 2 negative input Analog Input A4 MIC1+ Microphone 1 positive input Analog Input A5 MIC1– Microphone 1 negative input Analog Input B1 MODE0 Mic mode select pin Digital Input B2 MODE1 Mic mode select pin Digital Input B3 GA0 Pre-Amplifier Gain select pin Digital Input B4 GA1 Pre-Amplifier Gain select pin Digital Input B5 GND Ground Ground C1 MUTE2 Mute select pin Digital Input C2 GB0 Post-Amplifier Gain select pin Digital Input C3 NC No Connect C4 GA2 Pre-Amplifier Gain select pin C5 REF Reference voltage de-coupling Analog Ref D1 MUTE1 Mute select pin Digital Input D2 GB1 Post-Amp Gain select pin Digital Input D3 GB2 Post-Amp Gain select pin Digital Input D4 GA3 Pre-Amp Gain select pin Digital Input D5 VDD Power Supply Supply E1 LPF+ Low pass Filter for positive output Analog Input E2 OUT+ Positive optimized audio output Analog Output E3 OUT- Negative optimized audio output Analog Output E4 LPF- Low pass Filter for negative output Analog Input E5 SD Chip enable Digital Input www.national.com 4 Digital Input If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Supply Voltage Storage Temperature Power Dissipation (Note 3) ESD Rating (Note 4) ESD Rating (Note 5) CDM Junction Temperature (TJMAX) 235°C 70°C/W θJA (microSMD) Soldering Information See AN-1112 “microSMD Wafer Level Chip Scale Package.” 6.0V -85°C to +150°C Internally Limited 2000V 200V 500V 150°C Operating Ratings (Note 1) 2.7V ≤ VDD ≤ 5.5V Supply Voltage TMIN ≤ TA ≤ TMAX −40°C ≤ TA ≤ +85°C Electrical Characteristics 3.3V (Note 1, Note 2) Unless otherwise specified, all limits guaranteed for TA = 25°C, VDD = 3.3V, VIN = 18mVP-P, f = 1kHz, SD = VDD, Pre Amp gain = 20dB, Post Amp gain = 6dB, RL = 100kΩ, and CL = 4.7pF, f = 1kHz pass through mode. LMV1091 Symbol Parameter Conditions Typical Limits (Note 6) (Note 7) Units (Limits) VIN = 18mVP-P, A-weighted, Audio band 63 dB Signal-to-Noise Ratio VOUT = 18VP-P, voice band (300–3400Hz) 65 dB eN Input Referred Noise level A-Weighted VIN Maximum Input Signal THD+N < 1%, Pre Amp Gain = 6dB SNR VOUT Maximum AC Output Voltage DC Level at Outputs THD+N Total Harmonic Distortion + Noise ZIN Differential Out+, Out- μVRMS 5 880 820 mVP-P (min) 1.2 1.1 VRMS (min) 0.2 % (max) THD+N < 1% Out+, Out- 820 Differential Out+ and Out- 0.1 mV Input Impedance 142 kΩ ZOUT Output Impedance 220 Ω ZLOAD Load Impedance (Out+, Out-) (Note 9) RLOAD CLOAD AM Microphone Preamplifier Gain Range Minimum Maximum AMR Microphone Preamplifier Gain Adjustment Resolution AP Post Amplifier Gain Range APR Post Amplifier Gain Resolution 10 100 6 36 2 Minimum Maximum dB dB 1.7 2.3 6 18 3 kΩ (min) pF (max) dB (min) dB (max) dB dB 2.6 3.4 dB (min) dB (max) FFNSE Far Field Noise Suppression Electrical f = 1kHz (See Test Method) f = 300Hz (See Test Method) 34 42 26 SNRIE Signal-to-Noise Ratio Improvement Electrical f = 1kHz (See Test Method) f = 300Hz (See Test Method) 26 33 18 fRIPPLE = 217Hz (VRIPPLE = 100mVP-P) 99 85 dB (min) fRIPPLE = 1kHz (VRIPPLE = 100mVP-P) 95 80 dB (min) Input referred 60 2.0 dB dB Input Referred, Input AC grounded PSRR Power Supply Rejection Ratio CMRR Common Mode Rejection Ratio VBM Microphone Bias Supply Voltage IBIAS = 1.2mA eVBM Mic bias noise voltage on VREF pin A-Weighted, CB = 10nF IDDQ Supply Quiescent Current VIN = 0V 0.60 Supply Current VIN = 25mVP-P both inputs Noise cancelling mode 0.60 IDD 5 dB 1.85 2.15 V (min) V (max) μVRMS 7 0.8 mA (max) mA www.national.com LMV1091 Mounting Temperature Infrared or Convection (20 sec.) Thermal Resistance Absolute Maximum Ratings (Note 1) LMV1091 0.7 μA (max) Turn-On Time (Note 9) 40 ms (max) Turn-Off Time (Note 9) 60 ms (max) ISD Shut Down Current TON TOFF SD pin = GND 0.1 VIH Logic High Input Threshold GA0, GA1, GA2, GA3, GB0, GB1, GB2, Mute1, Mute2, Mode 0, Mode 1, SD 1.4 V (min) VIL Logic Low Input Threshold GA0, GA1, GA2, GA3, GB0, GB1, GB2, Mute1, Mute2, Mode 0, Mode 1, SD 0.4 V (max) Electrical Characteristics 5.0V (Note 1) Unless otherwise specified, all limits guaranteed for TA = 25°C, VDD = 5V, VIN = 18mVP-P, SD = VDD, Pre Amp gain = 20dB, Post Amp gain = 6dB, RL = 100kΩ, and CL = 4.7pF, f = 1kHz pass through mode. Symbol Parameter Conditions LMV1091 Typical Limit Units (Limits) (Note 6) (Note 7) VIN = 18mVP-P, A-weighted, Audio band 63 dB Signal-to-Noise Ratio VOUT = 18mVP-P, voice band (300–3400Hz) 65 dB eN Input Referred Noise level A-Weighted 5 μVRMS VIN Maximum Input Signal THD+N < 1% 880 820 mVP-P (min) Maximum AC Output Voltage f = 1kHz, THD+N < 1% between differential output 1.2 1.1 VRMS (min) SNR VOUT DC Output Voltage THD+N Total Harmonic Distortion + Noise ZIN ZOUT 820 Differential Out+ and Out- Input Impedance Microphone Preamplifier Gain Range AMR Microphone Preamplifier Gain Adjustment Resolution AP Post Amplifier Gain Range APR Post Amplifier Gain Adjustment Resolution 0.2 142 Output Impedance AM 0.1 mV Minimum Maximum kΩ 220 Ω 6 36 dB dB 2 Minimum Maximum % (max) 1.7 2.3 6 18 3 dB (min) dB (max) dB dB 2.6 3.4 dB (min) dB (max) FFNSE Far Field Noise Suppression Electrical f = 1kHz (See Test Method) f = 300Hz (See Test Method) 34 42 26 SNRIE Signal-to-Noise Ratio Improvement Electrical f = 1kHz (See Test Method) f = 300Hz (See Test Method) 26 33 18 fRIPPLE = 217Hz (VRIPPLE = 100mVP-P) 99 85 dB (min) fRIPPLE = 1kHz (VRIPPLE = 100mVP-P) 95 80 dB (min) Input referred 60 2.0 dB dB Input Referred, Input AC grounded PSRR Power Supply Rejection Ratio CMRR Common Mode Rejection Ratio dB 1.85 2.15 V ( min) V (max) VBM Microphone Bias Supply Voltage IBIAS = 1.2mA eVBM Microphone bias noise voltage on VREF pin A-Weighted, CB = 10nF Supply Quiescent Current VIN = 0V 0.60 IDD Supply Current VIN = 25mVP-P both inputs Noise cancelling mode 0.60 ISD Shut Down Current SD pin = GND 0.1 TON Turn On Time 40 ms (max) TOFF Turn Off Time 60 ms (max) IDDQ www.national.com 6 μVRMS 7 0.8 mA (max) mA μA Parameter Conditions LMV1091 Typical Limit Units (Limits) VIH Logic High Input Threshold GA0, GA1, GA2, GA3, GB0, GB1, GB2, Mute1, Mute2, Mode 0, Mode 1, SD 1.4 V (min) VIL Logic Low Input Threshold GA0, GA1, GA2, GA3, GB0, GB1, GB2, Mute1, Mute2, Mode 0, Mode 1, SD 0.4 V (max) Note 1: “Absolute Maximum Ratings” indicate limits beyond which damage to the device may occur, including inoperability and degradation of device reliability and/or performance. Functional operation of the device and/or non-degradation at the Absolute Maximum Ratings or other conditions beyond those indicated in the Recommended Operating Conditions is not implied. The Recommended Operating Conditions indicate conditions at which the device is functional and the device should not be operated beyond such conditions. All voltages are measured with respect to the ground pin, unless otherwise specified. Note 2: The Electrical Characteristics tables list guaranteed specifications under the listed Recommended Operating Conditions except as otherwise modified or specified by the Electrical Characteristics Conditions and/or Notes. Typical specifications are estimations only and are not guaranteed. Note 3: The maximum power dissipation must be de-rated at elevated temperatures and is dictated by TJMAX, θJC, and the ambient temperature TA. The maximum allowable power dissipation is PDMAX = (TJMAX – TA) / θJA or the number given in the Absolute Maximum Ratings, whichever is lower. For the LMV1091, TJMAX = 150°C and the typical θJA for this microSMD package is 70°C/W and for the LLP package θJA is 64°C/W. Refer to the Thermal Considerations section for more information. Note 4: Human body model, applicable std. JESD22-A114C. Note 5: Machine model, applicable std. JESD22-A115-A. Note 6: Typical values represent most likely parametric norms at TA = +25°C, and at the Recommended Operation Conditions at the time of product characterization and are not guaranteed. Note 7: Datasheet min/max specification limits are guaranteed by test, or statistical analysis. Note 8: Default value used for performance measurements. Note 9: Guaranteed by design. 7 www.national.com LMV1091 Symbol LMV1091 Test Methods 30092212 FIGURE 2. FFNSE, NFSLE, SNRIE Test Circuit two microphones (see Figure 9). In this configuration the speech signal at the microphone closest to the sound source will have greater amplitude than the microphone further away. Additionally the signal at microphone further away will experience a phase lag when compared with the closer microphone. To simulate this, phase delay as well as amplitude shift was added to the NFSLE test. The schematic from Figure 3 is used with the following procedure to measure the NFSLE. 1. A 25mVP-P and 17.25mVP-P (0.69*25mVP-P) sine wave is applied to Mic1 and Mic2 respectively. Once again, a signal generator is used to delay the phase of Mic2 by 15.9° when compared with Mic1. 2. Measure the output level in dBV (X) 3. Mute the signal from Mic2 4. Measure the output level in dBV (Y) 5. NFSLE = Y - X dB FAR FIELD NOISE SUPPRESSION (FFNSE) For optimum noise suppression the far field noise should be in a broadside array configuration from the two microphones (see Figure 8). Which means the far field sound source is equidistance from the two microphones. This configuration allows the amplitude of the far field signal to be equal at the two microphone inputs, however a slight phase difference may still exist. To simulate a real world application a slight phase delay was added to the FFNSE test. The block diagram from Figure 3 is used with the following procedure to measure the FFNSE. 1. A sine wave with equal frequency and amplitude (25mVP-P) is applied to Mic1 and Mic2. Using a signal generator, the phase of Mic 2 is delayed by 1.1° when compared with Mic1. 2. Measure the output level in dBV (X) 3. Mute the signal from Mic2 4. Measure the output level in dBV (Y) 5. FFNSE = Y - X dB SIGNAL TO NOISE RATIO IMPROVEMENT ELECTRICAL (SNRIE) The SNRIE is the ratio of FFNSE to NFSLE and is defined as: SNRIE = FFNSE - NFSLE NEAR FIELD SPEECH LOSS (NFSLE) For optimum near field speech preservation, the sound source should be in an endfire array configuration from the www.national.com 8 The overall noise of the LMV1091 is measured within the frequency band from 10Hz to 22kHz using an A-weighted filter. 30092211 FIGURE 11: Noise Measurement Setup For the signal to noise ratio (SNR) the signal level at the outtal gain (20dB preamplifier and 6dB postamplifier) with only put is measured with a 1kHz input signal of 18mVP-P using an Mic1 or Mic2 used. A-weighted filter. This voltage represents the output voltage The input signal is applied differentially between the Mic+ and of a typical electret condenser microphone at a sound presMic-. Because the part is in Pass Through mode the low-pass sure level of 94dB SPL, which is the standard level for these filter at the output of the LMV1091 is disabled. measurements. The LMV1091 is programmed for 26dB of to- 9 www.national.com LMV1091 The Mic+ and Mic- inputs of the LMV1091 are AC shorted between the input capacitors, see Figure 11. Measuring Noise and SNR LMV1091 Typical Performance Characteristics Unless otherwise specified, TJ = 25°C, VDD = 3.3V, Input Voltage = 18mVP-P, f = 1kHz, pass through mode, Pre Amp gain = 20dB, Post Amp gain = 6dB, RL = 100kΩ, and CL = 4.7pF. THD+N vs Frequency Mic1 = AC GND, Mic2 = 36mVP-P Noise Canceling Mode THD+N vs Frequency Mic2 = AC GND, Mic1 = 36mVP-P Noise Canceling Mode 30092257 30092258 THD+N vs Frequency Mic1 = 36mVP-P Mic1 Pass Through Mode THD+N vs Frequency Mic2 = 36mVP-P Mic2 Pass Through Mode 30092259 30092260 THD+N vs Input Voltage Mic1 = AC GND, f = 1kHz Mic2 Noise Canceling Mode THD+N vs Input Voltage Mic2 = AC GND, f = 1kHz Mic1 Noise Canceling Mode 30092261 www.national.com 30092262 10 LMV1091 THD+N vs Input Voltage f = 1kHz Mic1 Pass Through Mode THD+N vs Input Voltage f = 1kHz Mic2 Pass Through Mode 30092263 30092264 PSRR vs Frequency Pre Amp Gain = 20dB, Post Amp Gain = 6dB VRIPPLE = 100mVP-P, Mic1 = Mic2 = AC GND Mic1 Pass Through Mode PSRR vs Frequency Pre Amp Gain = 20dB, Post Amp Gain = 6dB VRIPPLE = 100mVP-P, Mic1 = Mic2 = AC GND Mic2 Pass Through Mode 30092265 30092266 PSRR vs Frequency Pre Amp Gain = 20dB, Post Amp Gain = 6dB VRIPPLE = 100mVP-P, Mic1 = Mic2 = AC GND Noise Canceling Mode Far Field Noise Suppression Electrical vs Frequency 30092268 30092267 11 www.national.com LMV1091 Signal-to-Noise Ratio Electrical vs Frequency 30092269 www.national.com 12 INTRODUCTION The LMV1091 is a fully analog single chip solution to reduce the far field noise picked up by microphones in a communi- 30092224 FIGURE 3. Simplified Block Diagram of the LMV1091 The output signal of the microphones is amplified by a preamplifier with adjustable gain between 6dB and 36dB. After the signals are matched the analog noise cancelling suppresses the far field noise signal. The output of the analog noise cancelling processor is amplified in the post amplifier with adjustable gain between 6dB and 18dB. For optimum noise and EMI immunity, the microphones have a differential connection to the LMV1091 and the output of the LMV1091 is also differential. The adjustable gain functions can be controlled via GA0–GA3 and GB0–GB2 pins. Bias microphone supply output pin depends on the noise voltage on the internal the reference node. The de-coupling capacitor on the VREF pin determines the noise voltage on this internal reference. This capacitor should be larger than 1nF; having a larger capacitor value will result in a lower noise voltage on the Mic Bias output. Gain Balance and Gain Budget In systems where input signals have a high dynamic range, critical noise levels or where the dynamic range of the output voltage is also limited, careful gain balancing is essential for the best performance. Too low of a gain setting in the preamplifier can result in higher noise levels while too high of a gain setting in the preamplifier will result in clipping and saturation in the noise cancelling processor and output stages. The gain ranges and maximum signal levels for the different functional blocks are shown in Figure 4. Two examples are given as a guideline on how to select proper gain settings. Power Supply Circuits A low drop-out (LDO) voltage regulator in the LMV1091 allows the device to be independent of supply voltage variations. The Power On Reset (POR) circuitry in the LMV1091 requires the supply voltage to rise from 0V to VDD in less than 100ms. The Mic Bias output is provided as a low noise supply source for the electret microphones. The noise voltage on the Mic 30092241 FIGURE 4. Maximum Signal Levels 13 www.national.com LMV1091 cation system. A simplified block diagram is provided in Figure 3. Application Data LMV1091 6. Calculating the new gain for the preamp will result in <23.5dB gain. 7. The nearest lower gain will be 22dB. So using preamp gain = 22dB and postamp gain = 6dB is the optimum for this application. Example 1 An application using microphones with 50mVP-P maximum output voltage, and a baseband chip after the LMV1091 with 1.5VP-P maximum input voltage. For optimum noise performance, the gain of the input stage should be set to the maximum. 1. 50mVP-P +36dB = 3.1VP-P. 2. 3.1VP-P is higher than the maximum 1.5VP-P allowed for the Noise Cancelling Block (NCB). This means a gain lower than 29.5dB should be selected. 3. Select the nearest lower gain from the gain settings shown in Table 2,28dB is selected. This will prevent the NCB from being overloaded by the microphone. With this setting, the resulting output level of the Pre Amplifier will be 1.26VP-P. 4. The NCB has a gain of 0dB which will result in 1.26VP-P at the output of the LMV1091. This level is less than maximum level that is allowed at the input of the post amp of the LMV1091. 5. The baseband chip limits the maximum output voltage to 1.5VP-P with the minimum of 6dB post amp gain, this results in requiring a lower level at the input of the post amp of 0.75VP-P. Now calculating this for a maximum preamp gain, the output of the preamp must be no more than 0.75mVP-P. www.national.com Example 2 An application using microphones with 10mVP-P maximum output voltage, and a baseband chip after the LMV1091 with 3.3VP-P maximum input voltage. For optimum noise performance we would like to have the maximum gain at the input stage. 1. 10mVP-P + 36dB = 631mVP-P. 2. This is lower than the maximum 1.5VP-P, so this is OK. 3. The NCB has a gain of 0dB which will result in 1.5VP-P at the output of the LMV1091. This level is lower than the maximum level that is allowed at the input of the Post Amp of the LMV1091. 4. With a Post Amp gain setting of 6dB the output of the Post Amp will be 3VP-P which is OK for the baseband. 5. The nearest lower Post Amp gain will be 6dB. So using preamp gain = 36dB and postamp gain = 6dB is optimum for this application. 14 The Pre-amplifier gain of the LMV1091TM can be controlled using the GA0-GA3 pins. See table 2 below for Pre-amplifier TABLE 2. Mic Pre-Amp Gain Settings GA3 GA2 GA1 GA0 Pre-Amplifier Gain 0 0 0 0 6dB 0 0 0 1 8dB 0 0 1 0 10dB 0 0 1 1 12dB 0 1 0 0 14dB 0 1 0 1 16dB 0 1 1 0 18dB 0 1 1 1 20dB 1 0 0 0 22dB 1 0 0 1 24dB 1 0 1 0 26dB 1 0 1 1 28dB 1 1 0 0 30dB 1 1 0 1 32dB 1 1 1 0 34dB 1 1 1 1 36dB TABLE 3. Post-Amp Gain Settings GB2 GB1 GB0 Post-Amplifier Gain 0 0 0 6dB 0 0 1 9dB 0 1 0 12dB 0 1 1 15dB 1 0 0 18dB 1 0 1 18dB 1 1 0 18dB 1 1 1 18dB Noise Reduction Mode Settings The LMV1091TM has four mode settings. It can be placed in noise cancellation mode, mic 1 on with mic 2 off, mic 1 off with mic 2 on, and mic1 and mic2. See table 4 for control settings. TABLE 4. Noise Reduction Mode Settings Mode 1 Mode 0 Noise Reduction Mode Selection 0 0 Noise cancelling mode 0 1 Only Mic 1 On 1 0 Only Mic 2 On 1 1 Mic 1 + Mic 2 15 www.national.com LMV1091 gain control. The Post-Amp gain can be controlled using the GB0-GB2 pins. See table 3 below for Post-amplifier gain control. Pre-Amp/Post-Amp Gains LMV1091 Mute Section Mic 1 and Mic 2 can be muted independently, using the Mute 1 and Mute 2 pins. See Table 5 for control settings. TABLE 5. Noise Reduction Mode Settings Mute 2 Mute 1 Mute Mode Selection 0 0 Mic 1 an Mic 2 on 0 1 Mic 1 mute 1 0 Mic 2 mute 1 1 Mic 1 and Mic 2 mute large, the far field noise reduction performance will be degraded. The optimum spacing between Mic 1 and Mic 2 is 1.5-2.5cm. This range provides a balance of minimal near field speech loss and maximum far field noise reduction. The microphones should be in line with the desired sound source 'near speech' and configured in an endfire array (see Figure 9) orientation from the sound source. If the 'near speech' (desired sound source) is equidistant to the source like a broadside array (see Figure 8) the result will be a great deal of near field speech loss. Microphone Placement Because the LMV1091 is a microphone array Far Field Noise Reduction solution, proper microphone placement is critical for optimum performance. Two things need to be considered: The spacing between the two microphones and the position of the two microphones relative to near field source If the spacing between the two microphones is too small near field speech will be canceled along with the far field noise. Conversely, if the spacing between the two microphones is 30092243 FIGURE 8: Broadside Array (WRONG) 30092242 FIGURE 9: Endfire Array (CORRECT) www.national.com 16 A-Weighted Filter At the output of the LMV1091 there is a provision to create a 1st order low-pass filter (only enabled in 'Noise Cancelling' mode). This low-pass filter can be used to compensate for the change in frequency response that results from the noise cancellation process. The change in frequency response resembles a first-order high-pass filter, and for many of the applications it can be compensated by a first-order low-pass filter with cutoff frequency between 1.5kHz and 2.5kHz. The transfer function of the low-pass filter is derived as: The human ear is sensitive for acoustic signals within a frequency range from about 20Hz to 20kHz. Within this range the sensitivity of the human ear is not equal for each frequency. To approach the hearing response, weighting filters are introduced. One of those filters is the A-weighted filter. The A-weighted filter is used in signal to noise measurements, where the wanted audio signal is compared to device noise and distortion. The use of this filter improves the correlation of the measured values to the way these ratios are perceived by the human ear. This low-pass filter is created by connecting a capacitor between the LPF pin and the OUT pin of the LMV1091. The value of this capacitor also depends on the selected output gain. For different gains the feedback resistance in the lowpass filter network changes as shown in Table 6. This will result in the following values for a cutoff frequency of 2000 Hz: TABLE 6. Low-Pass Filter Capacitor For 2kHz Post Amplifier Gain Setting (dB) Rf (kΩ) Cf (nF) 6 20 3.9 9 29 2.7 12 40 2.0 15 57 1.3 18 80 1.0 30092210 FIGURE 10: A-Weighted Filter 17 www.national.com LMV1091 Low-Pass Filter At The Output LMV1091 Revision History Rev Date 1.0 10/28/09 www.national.com Description Initial released. 18 LMV1091 Physical Dimensions inches (millimeters) unless otherwise noted 25 Bump micro SMD Technology NS Package Number TMD25AAA X1 = 2015mm X2 = 2015mm X3 = 600mm 19 www.national.com LMV1091 Dual Input, Far Field Noise Suppression Microphone Amplifier For more National Semiconductor product information and proven design tools, visit the following Web sites at: Products Design Support Amplifiers www.national.com/amplifiers WEBENCH® Tools www.national.com/webench Audio www.national.com/audio App Notes www.national.com/appnotes Clock and Timing www.national.com/timing Reference Designs www.national.com/refdesigns Data Converters www.national.com/adc Samples www.national.com/samples Interface www.national.com/interface Eval Boards www.national.com/evalboards LVDS www.national.com/lvds Packaging www.national.com/packaging Power Management www.national.com/power Green Compliance www.national.com/quality/green Switching Regulators www.national.com/switchers Distributors www.national.com/contacts LDOs www.national.com/ldo Quality and Reliability www.national.com/quality LED Lighting www.national.com/led Feedback/Support www.national.com/feedback Voltage Reference www.national.com/vref Design Made Easy www.national.com/easy www.national.com/powerwise Solutions www.national.com/solutions Mil/Aero www.national.com/milaero PowerWise® Solutions Serial Digital Interface (SDI) www.national.com/sdi Temperature Sensors www.national.com/tempsensors SolarMagic™ www.national.com/solarmagic Wireless (PLL/VCO) www.national.com/wireless www.national.com/training PowerWise® Design University THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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