XTR108: The output current is unchanged for XTR108

Hi Mickey,
Given the register contents you posted, I'd expect V/I-0 to be sourcing around 1.2mA. That would make the Vsensor voltage ~6.04V. Since the linearization constant is zero, this shouldn't change with changing input voltage. It looks like the bridge sensor's outputs should be connected properly to the internal PGA. Looking at the configured PGA gain (200x), and Rvi value (6.34k), I'd expect this configuration to output/draw ~19mA from the loop supply at full bridge loading. That current should be measured either at the supply (current into Vsup), or by inserting an ammeter at Header 2. Even at full output, the current out of V/I-0 should still be ~1.2mA, though.

Hello Alex ，
Thanks for your quick reply!
I still have problem with XTR108 which should output/draw ~19mA when I insert an ammeter at Header 2.
I'd like you to answer these questions：
1. can you explain why the V/I-0 should be sourcing around 1.2mA accroding to the register contents? I noticed 0.493mA with Rset=12.1k in the XTR108's datasheet. And I actrually got around 1mA from V/I-0 Pin.
2. when the output of bridge sensor is 8mV, the ammeter at Header2 show 2mA, not ~19mA which I would be so glad to see. This is the main prolem. can you tell me the possible reasons or which way I should go to solve it.
3. I am not sure whether my checksum is right, I just caculate it according to the datasheet. |Does it matter if the checksum is incorrect?

hi,can u help to urge your expert reply my question . this case is emergent

1. Here’s my calculation: First, from the XTR108 datasheet:

Substituting the formulae from table III, we get:

In the register map you show, N₁₁ is 0x50, and N₁₀ is 0x01, making their decimal values 80 and 1, respectively. V_ref is provided internally, and is 1.193V. R_set is 12.1k, as per your schematic.

Plugging in all those values gives Iref_1,2 = 616.3µA. Now, looking at the contents of register 9, we see that it’s set to 0x00. This means that both Iref₁ and Iref₂ will be connected to V/I-0 by the internal mux. This means that the actual current out of V/I-0 will be 2xIref_1,2 or 1.233mA. If the register map you posted displays N₁₀ and N₁₁ as decimal values already, I’d expect the current to be 1.140mA. This current flows into R12, so I'd expect to see its voltage (and V_sensor) at about 6.04V.

2. back-calculating the input voltage required for a 2mA output, I get an input voltage of -1.34mV. For the V to I stage of the XTR108, I calculate that a 253mV output from the PGA would yield this current, which is very close to the minimum output swing of the PGA. Have you tried switching your PGA’s polarity? Writing 0x32 to register 8 would do this.

3. It looks like your checksum is right. When I calculate it from the register contents, I come up with the same value as is in register 15. Additionally, register 0 is 0x00, meaning that the CSE bit is cleared. If the checksum were wrong, though, I’d expect the output to be around 1.77mA, as the datasheet indicates that the XTR108 will program itself to the lowest under-scale level in this event. 

Hi Alex ，
I am very grateful for your help!

About the question1, I agree with your caculating but I got the same Iref1,2 output at Pin V/I-0 no mater what value I write to the register 9 0x00 or 0x10 or 0x01. I got no output at pin V/I-0 if I write ohter value to register 9. So I asume that my setting has been writen to EEPROM and read out by the XTR108. This performace confused me. Can you think about that and give me more direction?

About question2, I have reversed the V/I-2 and V/I-3 to the PGA and just got same result. But a funny thing happened when I fed a 1.5v input to V/I-2 and V/I-3 by mistake, I measured 10.70mA at the meter, and when I adjust the input voltage the meter read out 10.6mA~ 10.8mA! looks like the PGA is not working properly. The output just always kept 2mA if the input to V/I-2 and V/I-3 is around 8mV .

Looking forwards to your comments

Best Regards,

Leo Zhao