OPA344: Degradation of performance characteristics in transition region in unity-gain configuration OPA344 vs. TLV2461

Michael

Part Number: OPA344
Other Parts Discussed in Thread: TLV2461, , OPA325

Dear all,

we are searching for a suitable OP-Amp for a unity-gain-buffer configuration to buffer the high-impedance source voltage signal which needs to be measured by an other device (RTU) with an ADC.

The design of this RTU is not optimized for measuring high-impedance sources, as the ADC in the RTU has a voltage devider on it's input in order to reduce input signal voltage to not exceed max ADC input voltage.

This voltage devider, however has a relatively low impedance, so that we need to buffer the output signal of our high-impedance source. Our rail voltages are 0 to 5 V, input voltage is also 0 to 5 V

We have now identified different options one is the OPA344, the other is the TLV2461.

Looking into the data sheet of the OPA344 I found that there is a transition region where characteristics may be degraded.

1) My question here is now, if this degradation is already included in the specs table, i.e. in the min and max values given there?

2) In the data sheet of the TLV2461 I cannot find information on such a transition region. So does this part not have this limitation and would probably be the better option for a unity-gain-buffer ?

Thank you for your support!

Best,

Michael

over 1 year ago

0 Clemens Ladisch over 1 year ago

Guru 268160 points

1. You have to look at the test conditions. For te OPA344, the offset voltage is specified for a single input voltage, V_S/2.

2. Rail-to-rail inputs need two transistors to cover the entire range, and thus always have a transition region. The exception are chopper-stabilized amplifiers, which null out the offset by regularly switching the inputs, and amplifiers that use a charge pump to generate a higher bias voltage so that a single transistor works.

Here is a search for "Zero Crossover" or "Zero Drift" amplifiers: https://www.ti.com/amplifier-circuit/op-amps/products.html#2192=Zero Crossover;Zero Drift. You probably want the OPA325.

+1 Jerry Madalvanos over 1 year ago

TI__Mastermind 26481 points

Hey Michael,

You can look for crossover distortion in the voltage offset vs common mode spec.

In the case of the TLV2461, it looks like there are two crossover regions.

If you want to avoid crossover distortion, Clemens is correct, the zero-crossover amplifiers utilize a charge pump to increase the voltage biasing the input transistor stage, and will allow you to use a single input pair instead of a complimentary nmos/pmos input stage.

Some chopper amplifiers will have a crossover distortion, but the apparent effects are minimized due to the offset nulling of the chopper architecture. If an amplifier is Zero drift and Zero Crossover, it will both utilize a charge pump for the inputs, and the chopping architecture to minimize offset in all conditions.

Here's the selection of cost-optimized zero-crossover devices: https://www.ti.com/amplifier-circuit/op-amps/general-purpose/products.html#2192=Zero%20Crossover&

If you'd like to see the full selection of TI's zero-crossover devices, see here:https://www.ti.com/amplifier-circuit/op-amps/products.html#2192=Zero%20Crossover&

Best,
Jerry

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OPA344: Degradation of performance characteristics in transition region in unity-gain configuration OPA344 vs. TLV2461