Internal Channels for DSA Devices
On a DSA device, you can either acquire a signal present on the I/O connector, or you can acquire a signal that is generated from the internal calibration multiplexer. The channels available on this multiplexer are typically used for calibration purposes, but you can also sample them as you would a physical signal present on the I/O connector. To read from one of these internal channels, you must use one of the device's AI physical channels (for instance, Dev1/ai0) when creating the channel to select which ADC to use, then set the appropriate string value on the Input Source channel attribute/property.
The following table lists internal channels for DSA devices.
| Internal Channel Name | Supported Devices | Description |
|---|---|---|
| _external_channel | All DSA devices | The source of the AI channel is the device input connector, or an accessory connected to the device connector. |
| _5Vref_vs_aignd | 446X, 447X | The source of the AI channel is the onboard reference signal (for example, +5V). |
| _ao0_vs_ao0neg* | 4461 only | The source of the AI channel is the onboard analog output channel 0. |
| _ao1_vs_ao1neg* | 4461 only | The source of the AI channel is the onboard analog output channel 1. |
| _aignd_vs_aignd | 446X, 447X, 449X | The source of the AI channel is the onboard ground signal. |
| _ref_sqwv_vs_aignd | 449X | The source of the AI channel is the onboard reference square wave signal. |
| * The AO internal channels are valid only for devices with AO physical channels. | ||
For all DSA devices, only one internal channel can be read at a time, although the same internal channel can be read on multiple physical channels (with additional restrictions for NI 447X devices). For example, you cannot simultaneously read the internal 5 V reference on one physical channel and the analog ground on another physical channel.
The NI 447X AI physical channels are grouped into pairs, for instance {ai0, ai1}, {ai2, ai3}, and so on. NI 447X devices cannot read an internal channel on more than one physical channel group, and when reading an internal channel, both physical channels in the group are connected to the internal channel source. For example, if the Input Source for channel ai0 is set to 5Vref_vs_aignd and the Input Source for channel ai1 is left at the default value of _external_channel, ai1 still reads the internal channel 5Vref_vs_aignd since ai0 and ai1 are in the same physical channel group.