D5a - 16 Channel DAC


Description

The D5a module contains 16 18-bit DACs (Digital to Analog Converters), initially created for setting gate-voltages on samples. In contrast to most commercial units, there are no processor or clock circuits present in this module. This to prevent interference. The only time any digital signals are present is when the DAC values are being changed. Afterwards the module is static again. The DAC values are set via the controller module. As this module is generally used for gate voltage control, the use of an isolated controller module (C1b/C2) is recommended.

The output voltage range can be set via software. These ranges are: ±4V, ±2V, +4V, ±8V and +8V. Use of the ‘high’ voltage mode (±8V and ) requires an additional power supply. Whatever the voltage range setting, the DAC resolution is always 18-bit. This allows the user to choose the optimal range, without sacrificing resolution.

D5a block diagram

Above is a block diagram of the inner workings of the module. There are 16 DACs that have a shared internal reference, selected for low temperature drift and low noise. In between the DACs and the reference is the circuit responsible for the emergency switch: ramping the voltage up and down slowly. This can be used in a power outage where the computer is down but the output voltages need to be ramped down slowly. It ramps all the DACs up or down in 5 to 10 seconds. When the switch gets turned back on, the DACs will return smoothly to the previously set value. The emergency switch has a mechanical (pull lever before operating) lock to prevent accidental switching. Rampdown time measurement: D5a Rampdown

Summing Option

To increase the resolution it is possible to sum up two neighbouring DACs, at the expense of halving the total number of DACs. This is a small hardware modification for which the unit needs to be modified. The figure below shows an example of the summing option. Here two neighbouring channels have been added, with the second channel attenuated by a factor hundred. Both are kept at ±4V span, which results in LSB steps of 300nV. However, this is below the noise of the output. The measurement has been done by taking the average of 400 of these traces.

The typical use case for this option is to set one DAC (the course DAC) at a wanted value, and the vary the other DAC (the fine DAC) around that value. In this use case the linearity is not an issue. It also has the added effect that any glitches due to updating the DAC are also divided by the factor hundred.

Specifications

Parameter Value Units Conditions
Output Characteristics
Range ±4, ±2, +4, (±8, +8) V ±8V and +8V only possible with additional power supply
Output Impedance 550 Ω
Analog Bandwidth 100 kHz Due to output filter
Maximum Update Rate 5 kSps With data stream from PC
250 kSps With data stream from controller module. Not yet implemented.
Static Performance
Resolution 18 bit
Monotonicity 18 bit
DNL ±0.2 LSB Typical value
INL ±0.5 LSB Typical value
Dynamic Performance
Slew Rate 1.4 V/μs
Rise/Fall Time 3.8 μs 10% to 90%
Overshoot 4%
Integrated Noise <4 μVpp 0.1 to 10 Hz bandwidth. Depends on output voltage.
Wideband Noise Density < 30 nV/√Hz @ 100 Hz
< 70 nV/√Hz @ 2 Hz
Temperature Drift -4 ppm/°C Scales with output voltage: @4V will be -16 μV/°C, @-1V will be 4 μV/°C
Miscellaneous
Ramp up/down time 5..10 s The safety switch ramp up/down time.
Supply Current 100 mA

Measurements

Output rise time, switching from -2 Volt to 2 Volt: D5a Rampdown