Hello: I'm trying to read differential voltages on my USB-6215 from 8 channels. I've looked at the example code, tried several configurations, but can't seem to get what I need. The short version is that no matter what I try, I can only seem to access the data for the first channel. I'm using AI0/AI8 as the input for channel 1 and AI1/AI9 for channel 2. In my test, I use two 1.6 volt AA batteries as test signals. No matter what I try, I can see the 1.6 volt input on channel 1 (Dev1/ai0), but can't see anything on the second channel (Dev1/ai1). I'm either not reading what I think I am, or I don't understand how to access the data in the array after the read. Any help would be greatly appreciated. The following code is my latest brute force try at reading multiple channels. I get no errors (i.e. result = 0) on all function calls, so I think it is just a matter of me not understaning the system: // === Analog Input Test ==================================== int result = 0; TaskHandle AnalogTaskHandle0 = 0;TaskHandle AnalogTaskHandle1 = 0; float64 data0[1000];float64 data1[1000]; char errBuff[2048]={'\0'}; int32 numSamplesPerChannelRead = 1; // Create the DAQ tasks for the analog reads.result = DAQmxCreateTask("",&AnalogTaskHandle0); result = DAQmxCreateTask("",&AnalogTaskHandle1); // Create the Channels result = DAQmxCreateAIVoltageChan(AnalogTaskHandle1,"Dev1/ai1","",DAQmx_Val_Diff,-10.0,10.0,DAQmx_Val_Volts,NULL);result = DAQmxCreateAIVoltageChan(AnalogTaskHandle2,"Dev1/ai2","",DAQmx_Val_Diff,-10.0,10.0,DAQmx_Val_Volts,NULL); result = DAQmxCfgSampClkTiming(AnalogTaskHandle0,"",10000.0,DAQmx_Val_Rising,DAQmx_Val_FiniteSamps,10); result = DAQmxCfgSampClkTiming(AnalogTaskHandle1,"",10000.0,DAQmx_Val_Rising,DAQmx_Val_FiniteSamps,10); int i = 0;while(TRUE) { for(i = 0; i < 100; i++) { data0[i] = 0.0; data1[i] = 0.0; } // Start the tasks result = DAQmxStartTask(AnalogTaskHandle0); result = DAQmxStartTask(AnalogTaskHandle1); // Read the channels result = DAQmxReadAnalogF64(AnalogTaskHandle0,10,0.005,DAQmx_Val_GroupByScanNumber,data0,1000,&numSamplesPerChannelRead,NULL); result = DAQmxReadAnalogF64(AnalogTaskHandle1,10,0.005,DAQmx_Val_GroupByScanNumber,data1,1000,&numSamplesPerChannelRead,NULL); // Stop the tasks. DAQmxStopTask(AnalogTaskHandle0); DAQmxStopTask(AnalogTaskHandle1); // See if any of the channels have a voltage greater than 1.0 V. for(i = 0; i < 100; i++) if(data0[i] > 1.0) TRACE("data0[%d]: %0.3f\n", i, data0[i]); for(i = 0; i < 100; i++) if(data1[i] > 1.0) TRACE("data1[%d]: %0.3f\n", i, data1[i]); } Bob H.

Multifunction DAQ

USB-6215 NI-DAQmx Analog Input Problems

What you are experiencing is a behavior known as ghosting. Essentially, the "infinite" input impedance of an unconnected channel does not provide a return path to ground for charge accumulated on the input capacitance of the ADC (remember, the 6215 is a multiplexed board meaning all of the channels are switched through a single ADC).

John Passiak