DAC input in feedback of Opamp.

• DAC input in feedback of Opamp.
  Posted by Sam__ on 17 Jan, 2014 12:05
• I just had a very stressful pre job interview question which was to analyse a circuit. I came across something I'd never seen before (exactly what you want to happen right?).
  
  There was a DAC input where there is usually a ground on a non-inverting amp.
  
  Here's the diagram.
  
  
  
  I was wondering what effect that has on the circuit and why it's useful?
• #1 Reply
  Posted by miceuz on 17 Jan, 2014 12:15
• errr.... to center the output of an opamp to a different point set by dac?
• #2 Reply
  Posted by ThomasDK on 17 Jan, 2014 16:15
• Quote from: miceuz on 17 Jan, 2014 12:15

  errr.... to center the output of an opamp to a different point set by dac?
  

  
  AKA adding an offset
• #3 Reply
  Posted by Sam__ on 17 Jan, 2014 16:17
• So the output of the opamp has an adjustable offset due to the DAC input?
• #4 Reply
  Posted by Jay_Diddy_B on 17 Jan, 2014 16:43
• Sam,
  Think of it this way:
  
  If pin 3 the positive input was grounded. You are left with the inverting amplifier configuration. From DAC3 to the op-amp output the gain is:
  
  Vout / DAC3 = -1 x R40/R43
  
  So this can be used for setting an offset.
  
  Look up 'superposition theorem'.
  
  Jay_Diddy_B
  
• #5 Reply
  Posted by Sam__ on 17 Jan, 2014 17:13
• Quote from: Jay_Diddy_B on 17 Jan, 2014 16:43

  Sam,
  Think of it this way:
  
  If pin 3 the positive input was grounded. You are left with the inverting amplifier configuration. From DAC3 to the op-amp output the gain is:
  
  Vout / DAC3 = -1 x R40/R43
  
  So this can be used for setting an offset.
  
  Look up 'superposition theorem'.
  
  Jay_Diddy_B
  

  
  Thanks a lot. I'll get reading!
• #6 Reply
  Posted by AlfBaz on 17 Jan, 2014 21:54
• Quote from: Sam__ on 17 Jan, 2014 16:17

  due to the DAC input?
  

  Is it any clearer if you called it a DAC output?
• #7 Reply
  Posted by Sam__ on 17 Jan, 2014 22:01
• Quote from: AlfBaz on 17 Jan, 2014 21:54

  Quote from: Sam__ on 17 Jan, 2014 16:17

  due to the DAC input?
  

  Is it any clearer if you called it a DAC output?
  

  
  output from the DAC... input to that part of the circuit from the DAC. all the same really. But I do understand what's going on now. Thanks a lot guys.
• #8 Reply
  Posted by cellularmitosis on 18 Jan, 2014 00:02
• it took my brain a moment to reason out why this creates an offset, rather than actually changing the gain.
  
  a naive conclusion would be that the DAC changes the gain, because the voltage divider created by the resistors is simply creating an input voltage, and the DAC can push that voltage around.
  
  however, a second glance reveals that the voltage divider is not creating a fixed voltage, it follows the AC signal.  since the DAC just offsets by a fixed amount, and does not follow the AC signal, it cannot be modifying the gain, only creating an offset.
  
  do I have that right?
• #9 Reply
  Posted by Rudane on 18 Jan, 2014 01:41
• The input to the inverting input will experience the gain -100k/100k which is -1. So, the DAC voltage gets inverted and adds a constant value to the output signal. The non-inverting input will experience the gain 100/100 + 1 or 2. So the output will be 2*Vin+ - DAC.