Amplifier 4

The fourth prototype amplifier consists of an Analog Devices AD623AN instrument amplifier IC and active 4th-order Butterworth lowpass filtration using a custom-made equal-component Sallen-Key filter. The lowpass cutoff for this amplifier is ~30 Hz, as this value is more appropriate for the acquisition of EKG as opposed to EEG signals, and it also reduces the amount of AC line noise acquired. The gain of the instrument amplifier chip is fixed at 1000x, although the equal-component Sallen-Key filter also adds a gain of about 2.5x, giving an overall gain of about 2500x.

Also in this design, the National Semiconductor MF4 discrete lowpass filter IC has been replaced with a custom-built continuous filter made from two op-amps and additional RC components. This replacement significantly improved the overall stability of the amplifier, and drastically reduced the inter-component noise caused by the switching of the discrete IC (and its power supply issues). All future designs will use custom continuous filters instead of turnkey ICs. At present, I strongly discourage the use of the MF4 or other discrete ICs in analog designs.

Schematic

Notes


Front side


Back side


Frequency response

The gain and phase response of this circuit at frequencies of interest (~2-80) Hz is shown here. The frequency response of external electronics can now be quickly and easily acquired using the Mac and component software instead of an external oscilloscope.


©Copyright Sky Coyote, 2001-2002.