Half the current in 200mA span and one third the current in 300mA span

Hi Divakaran ,

Thank you for reaching out! We're currently on holiday, but rest assured your question has been acknowledged and queued for a response as soon as we return. We appreciate your patience.

Br,

Den

Hi Divakaran 

What is the voltage output you see on the multimeter? 

You need a resistive load with correct power ratings to test.

We do not know what the resistive load is set or what voltage clamp is set while using the fluke multimeter in current mode.?

Hi Vsethia

I have added more details to my earlier post.

I have three experimental setups as described below.
Please refer the attached LTC2662 experimental setup diagram.

Note in the experiments below not all current sources were enabled simultaneously.

That is only one DAC current output was acting at any given time.

Setup 1.
Channel 0 current output is loaded by a 30 Ohm resistor and a Fluke 87 III multimeter in series.
The range of the multimeter is set to 400mA.

Setup 2.
Channel 1 current output is loaded by a Fluke 87 III multimeter only with range set to 400mA.

Setup 3.
Channel 4 current output is loaded by a 68mOhm (Rload) resistor and the voltage drop across this resistor is then amplified by
an instrumentation amplifier (IA), AD8222 whose gain is set to 991 (G). I am using the AD8222-EVALZ board for this purpose.
The AD8222-EVALZ board supply voltage +Vs = +15V, -Vs = -15V and both REF1 = REF2 = 0V (ground).

With the above mentioned setup the expected output voltage from the IA is given by:
Vout = Idac * Rload * G = Idac * 0.068 * 991 = Idac * 67.388.


The measured test results are also attached.

Based on the results the LTC2662 behaves correctly in the current ranges
3.125mA, 6.25mA, 12.5mA, 25mA 50mA & 100mA.

In the 200mA range the output current is 1/2 (half) the required value.
In the 300mA range the output current is 1/3 (one third) the required value.

Note - According to the multimeter specification its typical burden voltage in the above 400mA range is 1.8mV/mA.
That is typically 1.8mV will be dropped across the multimeter for every 1mA of current through it.

From these results it is clear that the LTC2662 can source the set current with a very low value load as in the case of the multimeter as load in the current measuring mode (Experiment 2) and the 68m Ohm (Experiment 3). The only exception being the 200mA and 300mA range.


Regards
Diva

PDFPDF

Hi Diva,

What are the voltages at the OUT pins in each of the test cases?

What is the clamp current set on each PVDD supply source?

What are the power ratings of each resistor used?

Hi Vsethia

The requested details provided below.

What are the voltages at the OUT pins in each of the test cases?
I have measured the voltage at the output pin for all three experiments using two methods.
Method 1 - Direct measurement at the output pin using a digital multimeter (Fluke 87 III).
These measurements are shown in bold, italic & purple in the attached table.

Method 2 - Using the LTC2662 MUX (Pin 28) to monitor output pin voltage.
These measurements are shown in bold, italic & brown in the attached table.

The measured voltages using methods 1 & 2 very closely agree for experiments 1 & 2.
But for experiment 3, 0.068Ohm load, the MUX output is not accurate but the direct measurement
of the voltage at the output is accurate as the the calculated current output closely
agrees with the set current value.


What is the clamp current set on each PVDD supply source?
The clamp current on the source power supply that powers all VDD pins is 3A.
Since I am using the DC2692A-A demo board V+ = 15V = VDD0 = VDD1 = VDD2 = VDD3 = VDD4.


What are the power ratings of each resistor used?
Experiment 1:
125mW for load resistor 30 Ohms axial metal film resistor, 1% tolerance.

Experiment 2:
This is internal to the Fluke 87 III multimeter and I am certain that it has adequate power rating
in the 400mA range. The 400mA range is also fused with a 400mA fuse and I have not blown it in any of the experiments.
Based on the multimeter typical burden voltage specification of 1.8mV/mA in the 400mA range the shunt resistor may be 1.8Ohms.

Experiment 3:
500mW for load resistor 0.068 Ohms 1206, 0.1% tolerance.

Regards
Diva

Hi Vsethia

Forgot to attach the measurements please find them below.

regards

Diva

PDF

Hi Divakaran,

You are using a load resistor with a power rating of 125mW.

Taking 100mA source current into account, the power dissipation in the load resistor is 100mA x 100mA x 30 Ohm = 300mW which is much above the power rating in your case. 

Also use the SDO echo readback to see what the span is being programmed in each case.  In this case, also check the value in the fault register.

Hi Vsethia

The cause of this issue is LTC2662 power limiting, as described in Table 15 on page 22 of the revision B of the data sheet.

"If VDDX − VOUTX > 10 V and the current range is ≥ 200mA, FR6 is set and the range for that channel is reduced to 100mA.

Can be disabled using the Config command (0111b).

This explains why I see half the current in the 200mA range.

I reduced the supply voltage accordingly to fulfil the above requirement and all works fine as expected.

Regards

Diva