XTR111: Amplifiers forum

HI Tom,

Yes, the XTR111 will show some ripple noise at the output, where the output ripple is proportional to the load current and load resistor. The XTR111 is able to achieve  DC precision/low drift by using an internal cycling chopper technique; and a small portion of the switching chopping noise glitches appear at the current output on the device. The accuracy of the current mirror relies on the dynamic matching of multiple individual internal current sources.  As you have mentioned, the datasheet discusses the chopper switching noise on the "Dynamic Performance" section on Figure 39 (without a filter), and recommends an output capacitor on figure 40, and a RC low pass filter on Figure 41 of the datasheet to reduce this noise.  

For relative low frequency 4-20mA current transmitter applications that require low noise, the RC low-pass filter configuration of Figure 41 is commonly used.  The corner frequency of the RC filter can be adjusted to reduce this noise with the trade-off in bandwidth.  The XTR111 is not recommended for applications that are sensitive to this noise while at the same time requiring a relative high bandwidth.  For example, the XTR111 is not compatible with current transmitter applications requiring HART communication, as the HART protocol has strict noise and frequency response requirements.

Please let me know if you have questions.

Thank you and Regards,

Luis

thanks for the info Luis. i have attached the filtered waveform from the datasheet

my question is, is this waveform generated by the ic or from the signal source feeding the ic?

the datasheet doesnt elaborate on the signal source so i assumed it would be cleaner than what is shown in figure 40.

Hi Tom,

Yes, the switching noise shown on the datasheet figure 40 above is generated by the XTR111 IC while applying a very low noise/stable DC input signal. 

If the XTR111 input voltage signal (VIN ) has additional noise, the output of the XTR111 will show the switching noise above plus the extrinsic VIN noise that is injected at the input of the device; where the VIN signal is gained up by the transfer function IOUT=10*VIN/RSET. 

Thank you and Regards,

Luis

Hi Luis. thanks for the reply. so, hypothetically, if the signal source was a static DC voltage with zero noise fed to the input of this ic, we would still see a ~40mv peak waveform shown in fig 40 appearing at the load resistor., correct?

Hi Tom,

Yes this is correct.  The device has an amount of intrinsic switch chopper noise even when applying a very low or ideal zero noise input signal.  Also, please review figure 6, p.6 of the datasheet showing the input-referred noise spectrum of the device, where the spectrum shows this switch noise components in the spectrum above 10kHz.

Thank you and Regards,

Luis 

HI Tom,

For clarification, if you require lower noise than figure 40 that shows around 40mV noise ripple, please check figure 41 where an additional RC filter has been added next to the load resistor to reduce the noise ripple to less than <5mV where noise is reduced significantly after adding the R=10kOhm and C=10nF filter.  The corner of the filter can be adjusted further noise reduction.

Thank you and Regards,

Luis

thanks Luis, the added filter does knock down the noise but the Fc is ~1.6khz which kills our bandwidth. are there other current transmitter ic's you recommend with lower noise than this one?

HI Tom,

Unfortunately, for 3-wire current transmitters, we only offer the XTR111, XTR300/5; they have a similar architecture that relies on dynamically-matched current mirrors what may show some residue of the internal chopping/switching.

An alternative is to build a discrete 3-wire current transmitter, similar to the one featured in the document below.  This is for a low voltage 5V supply application, high side current transmitter, using the OPAx333:

https://www.ti.com/lit/ug/slau502/slau502.pdf

The design procedure on the document above can be used to design similar higher voltage solutions by replacing the op-amps and transistors.  The second example below uses the OPA2197, which supports a higher supply voltage:

There are other topologies for voltage to current transmitters, depending on your needs.  What frequency range is required on this 4-20mA current transmitter application?

Thank you and Regards,

Luis

Hi Luis, thanks for the info. our target range is 0-20khz. our application tho is 2 wire. we would provide our customer with 2 connections: gnd and current output. they provide the termination resistor across these 2 connections and process the signal accordingly. hope this helps.

HI Tom,

The topology above could work as long as the load resistor is referred to the same GND potential used by the V to I converter circuit in a unipolar current output application. 

1) What is the voltage supply requirement, the input voltage amplitude, the required output current range, and load resistor range required in this application?

2) The frequency response is 0-20kHz, what is the noise requirement at the load resistor?

Note:  I have corrected the link to the document on the post above:

https://www.ti.com/lit/ug/slau502/slau502.pdf

Thank you and Regards,

Luis

hi Luis, thanks for the information. our requirement is to provide a current output from 0 to 25ma and a ground reference. the customer is required to terminate the current with a load resistance up to 500ohm, with 250ohm typical. for 5V across the resistor we would expect no more than 2mv ripple. as we have no control over the termination at the customer end, we cannot consider filters at that location. i will also look at your link. this looks like it might be more cost effective anyway.. hope this info helps.

Hi Luis, i forgot to include the voltage source info. the source is 0-5V so for 0-5V in, the output would be -25ma. we have available a 12V and 24V rail.

Hi Tom.

The V-I Current to voltage converter with the OPA2197 powered with a +12V supply will work while supporting the 250-Ohm load resistor with 25mA (6.25-V across the resistor).  If the load resistor is 500-ohm with 25mA (12.5V across the resistor), then will need to power with +24V supply for headroom. 

Please see the example below, I adjusted the Rs1 resistor to 2kΩ, to accept a VIN = 0-5V signal and produce a 0-25mA output.  The %error accuracy of the circuit is dependent on the resistor tolerance of Rs1, Rs2 and Rs3. 

Below is also the DC transfer characteristic VIN=0-5V, Iout=0-25mA, Rload=250Ω while powering the circuit with Vs=+12V. 

The noise is around ~258uVRMS or ~1.7mVpeak-to-peak (approximately) when Rload = 250-Ω , and Vout=5V, Iout=20mA.

Below is the transient simulation result with a 100mV to 5V sinusoidal signal at 20-kHz. (the VIN is slightly offset above GND to ensure the P channel FET is always on the simulation).

Below find the zip file with the TINA simulation files.

OPA2197_current_transmitter_forum_10-7-21.zip

Please let me know if you need anything.

Thank you and Kind Regards,

Luis