How is the internal Rsense applied in the AD5522's VI expansion scheme?

Hi stycle ,

This is to inform you that we have received your inquiry. I’m currently looking into this now and will get back to you shortly.

Regards,

Menchie

Hi stycle ,

Using internal current range, the AD5522 is designed for supply voltages that will not violate the given AVDD & AVSS conditions shown below. The maximum difference between the AVDD and AVSS is only +33V .

For example using asymmetric supplies, if your AVSS= -5V (the highest AVSS value you can use as per the specs below) then the maximum AVDD that you can have is +28V (33V-AVSS=+28V). Thus, you cannot force beyond +24V (+28V minus the headroom: 28V-4V=+24V).

It can’t support +/-36V using internal current range.

Hope this helps. Btw, we have a company shutdown, we’ll be back on Jan.4. If you want to pursue expansion of voltage range using external amplifier we can have internal team discussion, and we will reach out to you as soon as we come back.

Regards,

Menchie

Hi Menchie,

    Thanks for the reply. But we will only talk about the case of using an external VI expansion circuit.

    "Using Rsense to sample the currents that don't flow through it" seems to be what this question is trying to achieve. It may seem impossible, Is it possible to connect an external power amplification stage that can provide both current and voltage gain?

Best

Linss

Hi Linss,

Apologies for the delayed response.

To answer your inquiry, please take note that:

1.Any internal current range of AD5522 can't be used in voltage expansion ranging +/-36V as per the datasheet also explained in my previous response. Also, you cannot measure the current flowing in the external rsense using internal rsense resistor because current doesn’t flow through the internal rsense.

 2. Using external amplifier circuitry in AD5522 will use an external rsense resistor. 2uA current range can be configured using external circuitry. However, the voltage range possible at this current level is dependent on the voltage range of the external amplifier circuitry and gain.

This can be achieved by connecting external amplifier to EXTFOHx to gain up desired voltage output by gaining up the external amplifier used.

In the circuit shown below (please take note that this is just an example for conceptual discussion) using LTC6090-5 high voltage amplifier and gain setting of 4, forcing +/-9V to AD5522 will output +/-40V through the external amplifier. In this case, since we are expecting 2uA (example given) of current to go through the DUT, and as per specification of the AD5522 which requires a max voltage drop between the EXTMEASILx and EXTMEASIHx should not exceed 1.125V. In this case using 4V drop across external sense resistor and having it attenuated by four satisfies 1V differential voltage between the EXTMEASILx and EXTMEASIHx, an Rsense of 2Mohm is derived.

Rsense= 4V/2uA=2Mohm

RDUT=36V/2uA=18Mohm

 The voltage output going to the DUT then will be +/-36V.

LTC6091 op amp was added to fully isolate the DUT side to the measuring side and is intended for removing the gain of 4 from the voltage sense of the system going to AD5522 so that it will measure back to 9V. This must be done to satisfy the feedback loop that the voltage seen on the inverting and non-inverting side of force amplifier should always be equal.

You can try it first in breadboard and see if it works and fit your application. This might be feasible but total cost of system would increase and conditions must be done to fully utilize the potential of the external op amp while minimizing the loading effect and error in measurement block that will be encountered.

We cannot guarantee that using this circuitry will conform to the AD5522 characterization. The performance specs may be at risk and will not be the same as stated on the AD5522 datasheet.

Regards,

Menchie

Hi stycle Linss,

Could you share if you were able to perform the experiment or have any progress or outcome related to the discussion above? 

Regards,

Menchie