Instructions
This amplifier converts an analog voltage signal to a current. The input signal is referenced against ground. The dual amplifier configuration allows the amplifier to operate across the full supply voltage.
Schematic
![TIP TIP](http://bdml.stanford.edu/twiki/pub/TWiki/TWikiDocGraphics%20%20/tip.gif)
The voltage difference between V- and V+ must be at least 15 volts.
![ALERT! ALERT!](http://bdml.stanford.edu/twiki/pub/TWiki/TWikiDocGraphics%20%20/warning.gif)
The J1 and J2 connector are mislabeled on the schematic
Ripple suppression
The output displays a 100 mV pk-pk sine wave at 60 kHz. In order to remove this oscillation, a 0.1 uF capacitor is placed across resistor R2.
Trim Pot Tuning
Consider the combined resistance of resistor R3 and the trim pot as
![](http://bdml.stanford.edu/twiki/pub/RisePrivate/ImmersionCurrentDriver/_MathModePlugin_c1b2a9b9de127e04ad5d640a3ea49f1b.png)
. The trim pot must be adjusted such that:
Current Gain
The gain equation is
![](http://bdml.stanford.edu/twiki/pub/RisePrivate/ImmersionCurrentDriver/_MathModePlugin_7080f587e342e4c4ca6b86f078c0a048.png)
which gives approx. 1 amp/volt ratio for this amplifier.
Derivation
R8 is the current sensing resistor. Designate the output of the first op-amp as
![](http://bdml.stanford.edu/twiki/pub/RisePrivate/ImmersionCurrentDriver/_MathModePlugin_8fa48c74984e7ee904126908c6067967.png)
and the amplifier output at
![](http://bdml.stanford.edu/twiki/pub/RisePrivate/ImmersionCurrentDriver/_MathModePlugin_cb41dbe8930cd2b0a3b103f39743c3b9.png)
such that the voltage drop across R8 is
![](http://bdml.stanford.edu/twiki/pub/RisePrivate/ImmersionCurrentDriver/_MathModePlugin_383701864ce71ceb10fc48bd39e73bdf.png)
Also, designate the input signal as
![](http://bdml.stanford.edu/twiki/pub/RisePrivate/ImmersionCurrentDriver/_MathModePlugin_44b6bcf9b07ad439ef5242c5c25c3a69.png)
. Thus, for the op-amp inputs, we find that
![](http://bdml.stanford.edu/twiki/pub/RisePrivate/ImmersionCurrentDriver/_MathModePlugin_8fc13ebdc3000520a52a1828f23c74e4.png)
Using op-amp analysis, we assume
![](http://bdml.stanford.edu/twiki/pub/RisePrivate/ImmersionCurrentDriver/_MathModePlugin_bda13910b71f8fda52f37fb3ae771917.png)
and get
![](http://bdml.stanford.edu/twiki/pub/RisePrivate/ImmersionCurrentDriver/_MathModePlugin_a5fa12fac961897df0821cea5b936eb0.png)
We can re-arrange this as
![](http://bdml.stanford.edu/twiki/pub/RisePrivate/ImmersionCurrentDriver/_MathModePlugin_92e2105cfb7b49a443a976ea4d47d38c.png)
At this point choose
![](http://bdml.stanford.edu/twiki/pub/RisePrivate/ImmersionCurrentDriver/_MathModePlugin_a3c93958e1b44beeeca9f5a034b5adeb.png)
such that
![](http://bdml.stanford.edu/twiki/pub/RisePrivate/ImmersionCurrentDriver/_MathModePlugin_fc9ed49ab44ec4ea7c5576295f11e4c0.png)
(This is how we adjust the trim pot, this equation can be simplified to the trim pot equation)
Which simplifies the above equation to
![](http://bdml.stanford.edu/twiki/pub/RisePrivate/ImmersionCurrentDriver/_MathModePlugin_7abf3316c0bbefd694c3f418e001a803.png)
Using ohm's law across resistor R8
![](http://bdml.stanford.edu/twiki/pub/RisePrivate/ImmersionCurrentDriver/_MathModePlugin_76aa59baeb7763cb42ea3587011c3b2c.png)
Which can be re-arranged to find the gain equation.
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SalomonTrujillo - 28 May 2008