Just FYI, the dual OP-07 is costly... It is cheaper to put aside board space for two OP-07s. At 02:38 PM 1/18/2000 GMT, Mike Harrison wrote: >On Tue, 18 Jan 2000 07:54:14 -0500, you wrote: > >>Just look for the description "instrumentation amp" >>These with be hi gain, hi cmrr, low bias current and low offset voltage. >>The old standby is OP07 >>$.80 from digikey or mouser >>both Analog devices and linear devices will send you books on in-amps FREE >No - an instrumantation amp is a different animal - typically a >programmable-gain differential amplifier with buffered inputs and an >output reference connection. These are never cheap. >You need a 'precision' opamp. op-07 is probably cheapest but may not >be good at a 5V supply. >The main parameter of interest is the input offset voltage, which >effectively determines the accuracy. This needs to be less than your >required input measurement resolution. >Nat Semi and Texas do zillions of opamps, and I'm sure some will be >suitable. SGS-Thomson may also be worth a look - I don't know if they >do any precision parts but I've been pleasantly surprised by pricing >on some of their opamp parts in the past. >Also check out Analog devices, Burr-Brown, Linear Tech and Maxim, >although all these guys tend to be in the higher precision, higher >cost area. >At the higher-end you don't tend to get multiple devices - there's a >dual op-07 but that's about it. It's obvious why if you think about it >- they need to be selected for input offset, so if you need 2 good >amps on one die, the yield is going to be significantly less, and the >chances of getting 4 good ones are even lower! > If you only need one _precision_ amp and some vanilla ones, it's >usually cheaper to use a single plus a multiple cheaper package for >the others > > >>CSS >> >> -----Original Message----- >> From: Russell McMahon [mailto:apptech@CLEAR.NET.NZ] >> Sent: Tuesday, January 18, 2000 1:17 AM >> To: PICLIST@MITVMA.MIT.EDU >> Subject: Op Amp recommendatiuon sought >> >> I'm looking for an opamp that is "somewhat better" than the >>standard LM324 >> but still as cheap as possible. >> >> The application is a very simple one with a "real" opamp. >>Using a 324 it is >> a little challenging. >> >> The LM324 is very cheap - 4 opamps for about $US0.20 or so >>in volume but it >> has some very nasty problems with linearity when the output >>transits between >> its class A and class B modes. There are many duals and >>quads but few come >> close to the 324 in price - someone may have a favourite >>recommendation. The >> circuit is desirably a single supply one but if a cheap >>enough dual supply >> opamp was available I could consider generating a negative >>rail. >> >> Application: Monitor current sense resistor in source of >>a FET (or IGBT) >> and translate the current into a voltage of about 0 to 4 >>volts. >> Current is up to about 5 amps, so to avoid excessive power >>dissipation >> (which is not totally critical) the resistor should be a >>maximum of about >> say 0.01 to 0.1 ohms. (0.1r = 2.5 watt dissipation at peak >>power). Voltage >> then = 0.5 volt. >> Therefore a gain of about 4/0.5 = 8 x is required. Not a >>very demanding >> application. There is some AC noise on the current so a >>filter is required >> to produce a smooth DC mean - this could be a 1 pole >>prefilter or, >> preferably, the gain stage would be an active filter. >> The current is ground referenced and the output is ground >>referenced. >> The output of the amplifier drives a resistor of a few >>hunded ohms which has >> a variable DC voltage and a significant high frequency AC >>component. This >> point MUST see the amplifier output as a very low impedance >>point. >> >> Using a 324 horrendous things happen. The cross over with >>increasing voltage >> is "interesting". Driving a load which has its "other" end >>above the 324 >> output leads to unexpected behaviour. >> >> SO - >> >> Any favourite amplifiers with the following specs. >> >> - MUST be cheap. MUST be. >> - 5 volt single supply operation preferred but not >>essential. >> - Dual or quad preferred (single OK if price appropriate) >> - Output must drive to ground (but not +ve rail) >> - Input common mode needs to include ground.(but not +ve >>rail) >> - Did I say that it must be cheap? >> - Must behave "normally" when driven in a "normal" manner >>(unlike a 324 :-)) >> - Essentially only a DC amplifier but must provide low >>impedance sink to 100 >> KHz odd signal on load. >> >> AMP >> >> A 270r say >> ---AA >> AAA------RRRRR---- DC + 100 KHz >> ---AA >> A >> >> >> TIA >> >> >> >> Russell McMahon >> _____________________________ >> >> From other worlds - www.easttimor.com >> www.sudan.com >> >> What can one man* do? >> Help the hungry at no cost to yourself! >> at http://www.thehungersite.com/ >> >> (* - or woman, child or internet enabled intelligent entity >>:-)) > > ----------------------------------------------------------------- Sent from the desk of: Terry Allen Steen, EE engineering@marinapower.com 332 McLaws Circle, Ste 111 757-258-8800 (Voice) Williamsburg, Va 23185 757-258-8805 (FAX) ----------------------------------------------------------------- !I AM A WHALE MAIL USER! 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