# Diagnosing a Broken summing amplifier [closed]

My spec is a DAC, has to take input and output 0-4 volts, 5 bit input. It seemed logical to use a 741 IC set up as a summing amplifier, with gains of 1/2, 1/4, 1/8, 1/16, 1/32. I built it and got to testing, only do find it doesn't work and I'm not sure why: diagrams Apologies for just sending a link - am on mobile.

I checked all the resistor values and the smallest is 2150 ohms and with calculation they all give the right gains plus or minus a percent, but when I put 5 volts into the row that has a gain of 1/2, it outputs 0.39 volts... this is clearly not correct and would appreciate help understating why

• Those aren't diagrams. – Scott Seidman Jun 29 '17 at 14:12
• You might want to read electronics.stackexchange.com/questions/304521 – PlasmaHH Jun 29 '17 at 14:20
• You need to provide a true schematic. Also, what are your supply voltages? The 741 requires fairly high supply voltages to work properly. – Null Jun 29 '17 at 14:21
• @Cursed1701 Minimum recommended supply voltage is +/-10V (i.e. 20V) but you're only giving it 8V. – Null Jun 29 '17 at 14:33
• @Cursed1701 - Null is correct. A 741 will never work with +6/-2. Try +/- 12. Also, just for what it's worth, your second row of resistors, connected to the summing point with a blue wire, is not connected. The blue is one row too high. – WhatRoughBeast Jun 29 '17 at 16:02

I'm shouting because you failed to study the datasheet thoroughly and ask questions about what does $Vin_{CM}~~$ mean. It means Vin(+) must be self-biased to Vcc/2 and used as your virtual and analog ground.