OPA209: OPA209 output offset

Hi Red,

is the OPA209 supplied by a single supply? Then you violate the common mode input voltage range when 12Vb is 0V. The input voltage must be 1.5V more positive than the negative supply rail.

Kai

Hi Red,

Kai's observation is indeed correct if a single V+ supply is being used. Please tell us what power supply levels are being provided to the OPA209 V+ and V- pins? Also, what are the values of the four resistors connected to the OPA209?

Regards, Thomas
Precision Amplifiers Applications Engineering

Hi Kai, Thomas, 

The supply voltage on OPA209 is V+ = 18V, V- = GND

thanks a lot 

Hi Red,

Thank you for providing the power supply and resistor values for the OPA209 circuit.

There is a common-mode voltage (VCM) violation condition created when the OPA209 is operated with a single, +18 V supply and the 12 V supply is turned off. The Rshunt resistor nodes float under that condition and the 205 k resistor connected to the OPA209 non-inverting resistor pulls the inputs close to ground (0 V). It won't be exactly 0 V because of the input bias currents flowing through the resistor, but it will be close.

The OPA209 lower end VCM limit is (V-) + 1.5 V, or +1.5 V because V- = 0 V. Pulling the inputs near 0 V and below +1.5 V violates this specification. Once the linear VCM range is exceeded the op amp output pulls up against an output swing limit. Since the OPA209 output is unloaded it will rest several hundred millivolts above the lower negative output rail (0 V).

This can be corrected by adding a negative voltage of a few volts to the OPA209 V- pin; however, that isn't always practical or cost effective. Alternately, a rail-to-rail I/O op amp such as the OPA197 has a lower VCM limit of (V-) - 0.1 V, and the output when unloaded can move down to 25 mV, or less. Note that the output linearity is much degraded in the output saturation region and doesn't truly become linear until the output is several hundred millivolts above V- (0 V). That is the price to be paid if a V- supply can't be used.

You can view the OPA197 datasheet here:

www.ti.com/.../opa197.pdf

Regards, Thomas

Precision Amplifiers Applications Engineering

Dear support team 

Thanks for your reply. I’ve several questions would like to know

 

1. As I review OPA209 datasheet, I’ve seen figure 14 in OPA209 datasheet. Nonetheless, I haven’t seen the similar graph in OPA197 datasheet. Would you please help to provide the OPA197’s graph?

Note. We would like to know OPA197 input offset voltage to V_CM performance if V- = 0V

2. As for OPA197, I’ve seen input offset voltage to V_CM in figure 6. However, figure 8 shown the zoom in part of V_CM = 0 V. Why the input offset voltage near to V_CM = 0 V is quite different between these two figure. What is the difference?

 

3. “Note that the output linearity is much degraded in the output saturation region and doesn't truly become linear until the output is several hundred millivolts above V- (0 V).” Which part in the spec could we know the output saturation region?

 

Hi Red,

Your questions:

1. As I review OPA209 datasheet, I’ve seen figure 14 in OPA209 datasheet. Nonetheless, I haven’t seen the similar graph in OPA197 datasheet. Would you please help to provide the OPA197’s graph?

Note. We would like to know OPA197 input offset voltage to V_CM performance if V- = 0V

Unfortunately, a similar Offset Voltage vs Common-Mode Voltage curve was not developed for the OPA197. But keep in mind that even though the OPA197 VCM range includes 0 V when the operated with a single +12 V supply the output swing to the rail will be the limitation and that voltage limit will appear at the output. Therefore, the very low input offset value will not be achieved under that condition.

You can learn more about the subject of op amp input and output limitations here:

1130 - Input and Output Limitations.pptx

One way to get around the output swing limitations when using a single supply is to employ a low-noise negative charge pump to provide a low-level V- supply. Doing so would allow the OPA197 output to truly swing to 0 V, and even a bit negative while maintaining output linearity and very low output referred voltage offset. The recommended charge pump for the application is the LM7705. Here's the datasheet:

lm7705.pdf

2. As for OPA197, I’ve seen input offset voltage to V_CM in figure 6. However, figure 8 shown the zoom in part of V_CM = 0 V. Why the input offset voltage near to V_CM = 0 V is quite different between these two figure. What is the difference?

OPA197 Figure 6, Offset Voltage vs Common-Mode Voltage is for the power supply conditions of +/-18 V. You will note that the positive VCM doesn't exceed +15 V so the input stage crossover region is approached. Therefore, you do not observe an input crossover condition.

Figure 7, Offset Voltage vs Common-Mode Voltage raises the positive VCM up to +18 V so the crossover region is reached and the offset ripple is observed.

Figure 8, Offset Voltage vs Common-Mode Voltage is actually for +/-2.5 V supplies and with VCM being -2.5 V to +2.5 V the crossover region is evident. The graph is missing the +/-2.5 V label.

3. “Note that the output linearity is much degraded in the output saturation region and doesn't truly become linear until the output is several hundred millivolts above V- (0 V).” Which part in the spec could we know the output saturation region?

More so one should be looking at the region where the op amp is specified as being in linear operation. That is evident from the open-loop voltage gain (Aol) specifications in the Electrical Characteristics table. Aol provides the linear output voltage swing range associated with different supply and load conditions. If the op amp is operated within those stated conditions the output will be in the linear operating region and saturation will be avoided.

Regards, Thomas

Precision Amplifiers Applications Engineering