Ideally, the signal to noise ratio should be at least 50 to 1 to produce a quality output signal. A good signal to noise ratio can be achieved in one of two ways: • Boost the output signal by increasing the gain. • Reduce the noise. While increasing the gain is the simplest solution, too much gain can impose a limitation on the dynamic range of the amplifier. Reducing noise is a more complicated solution, but it offers a greater range and more stability in the end. Two-Stage Amplifiers Bio-amplifiers usually involve multiple stages of amplification. Stage One – The unadulterated signal coming into the amplifier is unaffected by the intrinsic noise of the amplifier. Then, it runs through the critical first stage of amplification where the signal is boosted by the primary gain factor to produce an output signal with the desired signal to noise ratio. The intrinsic noise is not amplified in the first stage. Higher gain factors used in the first stage of amplification can seriously limit the dynamic range available at output stage. Large stage one gains also limit the gain factor available in the second stage of amplification. Stage 2 – The stage one output signal enters the second stage of amplification where both the signal and the noise from the first stage are amplified together by the second stage gain factor so that the signal is large enough to be seen on a chart recorder or data acquisition system. The second stage amplification is the gain the user controls. It does not change the signal to noise ratio. Result of Amplifier Using Gain to Control Signal to Noise Ratio +5.0V +5.0V +5.0V -5.0V -5.0V -5.0V Output signal viewable in range Offset causes the signal peak to hit the upper rail Increasing offset renders the signal useless Fig. 2–As the offset naturally increases over time, a poorly constructed amplifier will not be able to faithfully reproduce the signal. This offset can also be a result of gain drift which can occur as the temperature rises. Instead of using high gains in the first stage of amplification, a well constructed bio-amplifier that uses high quality components, like WPI’s DAM series amplifiers, minimizes the noise in the first stage of amplification so that the dynamic range is retained throughout the amplification process. A poorly designed amplifier will simply increase the gain of the first stage amplification until the desired signal to noise ratio is reached. beyond ±10.0V. Since the industry standard limits us to ±10.0V power supply rails, the only way to improve the signal to noise ratio is to minimize the shot noise in the first stage of amplification. This is why high quality amplifier components are imperative. Why does my signal flatline? Regardless of the amplifier used, biological potentials are often accompanied by a DC offset, because the electrodes polarize over time. The DC offset naturally increases over time. Since the poorly constructed amplifier that utilizes greater first stage gain has restricted its dynamic range, it has limited ability to handle this offset. As the offset continues to increase, the output signal may eventually be forced by the offset into the rail causing the flat line (clipping the signal). (See Fig. 2.) AMPLIFIERS, ELECTROMETERS Why not boost the power rails? Theoretically, increasing the voltage rails powering the amplifier will increase the available dynamic output range. It would seem natural to increase the power supply rails coming into the amplifier in order to provide the capability for greater first stage gains. However, most data acquisition systems are limited to a maximum input signal ranging between ±10.0V. Therefore, it is not practical to increase the power rails of bio-amplifier Amplifier AC/DC Differential Headstage EMG EKG StimuIsolated lation Multichannel Battery Powered The amplifier that minimizes the noise in the first stage amplification offers a larger dynamic output range and handles a much greater offset Connectors value. Intracellular Bioamplifiers FD223A Electro 705 Duo773 DC DC DC t t t t t t 2 2 t 2 mm pin 2 mm pin 2 mm pin WPI’s amplifiers The purchase of a low-noise amplifier pays dividends in the end. WPI’s amplifiers were engineered for the bio-medical researcher. While 20-30μV of noise is common in bio-amplifiers, WPI’s DAM series amplifiers generate 0.4μV RMS (root mean squared) at 0.1-100Hz. (That’s equal to 2μV peak-to-peak.) The chart at left compares WPI’s bio-amplifiers. Extracellular Bioamplifiers ISODAM8A ISO80 DAM50 DAM80 DC AC AC/DC AC t t t t t opt t t t t t t t t t 4-8 t t t Mini Banana or 8-pin DIN Mini Banana RJ-11 Mini Banana Transducer Amplifiers BRIDGE8 TBM4M DC DC t t 4-8 4 8-pin DIN WPI transducers 8-pin DIN WPI transducers See www.wpiinc.com/amplifiers Epithelial Voltage/Current Clamp Bio Amplifier EVC4000 DC t 1-4 Ussing 2 mm ORDER TOLL-FREE: 866-606-1974 (U.S. only) World Precision Instruments • Tel: 941-371-1003 • Fax: 941-377-5428 • E-mail: sales@wpiinc.com • Internet: www.wpiinc.com 73