Verifying Simultaneous DAC Channel Updates with LDAC

Writing several DAC channels over a serial bus does not necessarily update their outputs at the same time. Many devices provide input registers and a separate update event so values can be staged before being applied together. Verify the command behavior and latch configuration rather than assuming that closely spaced writes are simultaneous.

Map the two register stages

The Analog Devices AD5686 and AD5684 data sheet distinguishes input-register writes, DAC-register updates, combined commands, and LDAC behavior. Channel masking can change which outputs respond to the shared update event.

Record the command used for each channel and the state of the hardware update pin. Identify any channel configured to update immediately. A single immediate-update command can break an otherwise coordinated sequence.

Load distinguishable values

Use different initial and final values on each channel so missed writes or channel-address mistakes are visible. Hold the update event inactive while loading all intended input registers.

Observe both the serial transaction and analog outputs. Confirm that staging writes do not change outputs prematurely under the actual configuration. Read back registers where supported, while remembering that register agreement alone does not measure analog timing.

Capture the common update

Apply the update event with its specified timing relative to the serial interface. Measure the event and all relevant outputs on a common timebase. Account for probe and channel skew before interpreting small differences.

Define what simultaneous means for the application. The beginning of an output transition and the time each output enters its final error band are different metrics. Different step sizes or loads can produce different settling times after the same digital latch event.

Exercise partial and interrupted sequences

Check the intended behavior when only some channels are updated. Verify that masks and retained values match the application design. Include startup, reset, and software restart, which may restore default latch settings.

For example, a control pair may require both values to change together. If a transaction fails after loading the first value, the software needs a defined response before issuing the shared update. Test that sequence without allowing a mixed pair to pass unnoticed.

Repeat at the maximum supported update rate and with the actual loads. Confirm that the bus can finish staging the complete group before the next scheduled latch event.

Save the command sequence, mask settings, timing requirements, and analog captures. The resulting evidence should show that the intended group updates coherently and reaches acceptable output values in time. A shared LDAC wire establishes a control mechanism; the configured commands and measured outputs establish its behavior.