Hi everyone!

I am making an amplifier for photodiode with OPA-128, a circuit is also available in application notes in datasheet (Figure 7). Circuit is attached
I am confused about the "Guard" at inverting pin of op-amp. how to implement this? Does this mean to connect pins 8,2,and GND together? It doesn't make sense to me.

P.S: I am new in the field of electronics, so sorry if my question is of basic type.

 

Only 8 (the "can" of the part) and GND (ground) are connected together. "Ground" here means circuit common. There is no implication of an actual earth ground, though in a circuit like this such a connection may be made for shielding purposes.

The guard is a narrow (usually, but can be wide) track on the printed circuit board that surrounds pins 2 and 3 and the things connected to them electrically. It should be done on both sides of the circuit board.

The reason for this is that the inputs of the amplifier are extremely high impedance but operate, by virtue of the behavior of the op-amp, at "virtual ground." The non-inverting input (NII) is connected to ground through an extremely high value resistor. If the op amp were perfect, the NII would be connected directly to ground, but even the minute current that flows into or out of the inputs causes errors at such high impedance so the resistor is used to "balance" error. The inverting input is maintained at the same voltage as the NII by the current through the feedback resistor. When impedances are super-high, the slightest bit of conductance to anything else from the high impedance nodes can cause significant errors. The guard ring prevents conductance across the surfaces of the circuit board to anything other than actual circuit ground. Because the two inputs of the amp are also at ground potential, no current will flow between the guard and the input pins.

In other circuits a guard might be connected to something other than ground because the node(s) being guarded operate at some voltage other than ground. For example, in a non-inverting amplifier the guard might connect to the output of the amp, directly for a gain-of-1 amp or through voltage divider resistors for other gains. Sometimes another amp is used to drive a guard. But the idea is always the same - block the possibility of current flowing to the "wrong" thing by surrounding nodes with copper at or very close to the voltage at which the nodes operate. Of course some very small amount of current flowing into or out of the guard must not change the voltage on the guard - it needs to be reasonably low impedance.

Extreme cleanliness of the circuit board is critical to operation of a circuit like this. All soldering flux must be removed which will involve solvents to remove both the base material of the flux ("rosin" for the older type of flux most commonly used) and the "activators" that make the flux effective. This can take multiple washes in an organic solvent such as an alcohol followed by wash with water with suitable detergent then multiple careful rinses with distilled water. Circuit boards are often baked at moderate temperature to dry them thoroughly after cleaning and may be coated with "conformal coating" but that is something that must be evaluated carefully.

The high-value resistors shown are not common and may be hard to obtain and quite expensive. Again, scrupulous cleanliness is required to prevent the slightest conduction over the surfaces of the parts and circuit board. "Exotic" material such as Teflon might be required for the circuit board substrate. What would be a simple inexpensive circuit at a impedances of a megohm ohms becomes difficult and expensive at 100 giga-ohm impedance - or more appropriately in terms of the application, at femtoampere currents.

AN-241 from National Semiconductor (Texas Instruments) will explain in more detail. It was written nearly forty years ago but the principles are still the same. You can find other similar info on guarding and shielding in high impedance circuits.

 

Ref material -

http://www.analog.com/en/technical-articles/layout-for-precision-op-amps.html

http://ww1.microchip.com/downloads/cn/AppNotes/cn540604.pdf

http://www.ti.com/lit/an/snoa664/snoa664.pdf

Regards, Dana.

 

@ebp
Thank you so much for a detailed response. (Though i was confused about "GUARD" portion only, i really appreciate your effort to explain complete circuit)

@danadak Thank you too, documentation especially 3rd file helped a lot