USB-6451 Block Diagram

The following figure shows the analog input circuitry of the USB-6451.

Every differential channel pair has its own instrumentation amplifier, multiplexers, and ADC. This allows high-speed simultaneous sampling of all differential channels up to the maximum rate of 1 MS/s on all channels. For additional flexibility, multiplexers allow sampling the differential input pair as 2 separate single-ended channels. In this single-ended mode, the maximum sample rate drops to 500 kS/s on all channels.

I/O Connector—You can connect analog input signals to the USB-6451 through the I/O connector.

AI Terminal Configuration—These multiplexers configure the input mode to either differential, referenced single-ended (RSE), or non-referenced single-ended (NRSE).

Instrumentation Amplifier (NI-PGIA)—The NI programmable gain instrumentation amplifier (NI-PGIA) can amplify or attenuate an AI signal to ensure that you get the maximum resolution of the ADC. The NI-PGIA also allows you to select the input range.

ADC—The analog-to-digital converter (ADC) digitizes the AI signal by converting the analog voltage into a digital number.

Analog Input Timing Signals—For information about the analog input timing signals available on USB-6451 devices, refer to the Analog Input Timing Signals section.

AI FIFO—The USB-6451 can perform both single and multiple A/D conversions of a fixed or infinite number of samples. A large first-in-first-out (FIFO) buffer holds data during A/D conversions to ensure that no data is lost. The USB-6451 can handle multiple A/D conversion operations with DMA or programmed I/O.

The main blocks featured in the USB-6451 analog output circuitry are as follows:

Digital-to-analog converters (DACs)—Convert digital codes to analog voltages.

AO FIFO—Enables analog output waveform generation. It is a first-in-first-out (FIFO) memory buffer between the computer and the DACs. It allows you to either stream a waveform with new data continuously provided from USB or download an entire waveform to the USB-6451 where it can be regenerated entirely from the onboard buffer.

AO Sample Clock—Reads a sample from the AO FIFO and generates the AO voltage.

These signals can function as digital I/O as well as counter, timer, or triggering I/O. When used as counter, timer, or triggering I/O, the lines are referred to as Programmable Function Interface (PFI) lines. The digital I/O lines on the USB-6451 support the following features:

• Direction and function of each terminal individually controllable
• Static digital input and output
• High-speed digital waveform generation
• High-speed digital waveform acquisition
• Digital input change detection trigger/interrupt
• Timing input signal for analog input, analog output, digital input, digital output, or counter/timer functions
• Timing output signal from analog input, analog output, digital input, digital output, or counter/timer functions
• Shared I/O voltage logic family selection: 1.8 V, 2.5 V, 3.3 V, or 5 V (shared for all lines)

The following figure shows the circuitry of one digital I/O line. Each digital I/O line is similar.

In software, these channels are referred to as port0/line0:15 when used as digital I/O and PFI 0:15 when used for other purposes, such as timing I/O.