ADC-input conditioning (II)

The previous version of this page.


The output of the CCD – without any circuitry – is app. 0.6V-2.5V. When placed in the drive circuit from in datasheet the output voltages are app. 1.2V-3V.

Neither of them is good match for the range of the ADC in the STM32F401RE – and then there’s the annoying thing with the inverse relationship between exposure and output-voltage.

One solution to this problem is to use an opamp in inverting mode before the ADC, to invert and scale the output from the CCD to match the 0-3.3V input range of the STM32F4’s ADC. I’ve used the AD8027, which is a high speed rail-to-rail opamp, in a circuit like this:

The ±4.65V supply voltages are generated from the USB’s 5V. A charge pump inverts the supply voltage to -5V, and the positive and negative rails are regulated down to the ±4.65V by two low-noise low drop-out regulators.

The gain is:

G = - R₂ / R₁

we have an input range of 1.9V, but want 3.2V (actually 3.3V but I don’t want to saturate the ADC-input), so G should be -1.68.

The level shift is:

S = R₄/(R₃+R₄)·(1+R₂/R₁)·Vref

The output voltage then becomes:

Vout = G·Vin + S = -R₂/R₁·Vin + R₄/(R₃+R₄)·(1+R₂/R₁)·Vref

The following values gave an output close to the limits of the ADC’s range, when using the TCD1304 in the typical drive circuit (supply voltage 4V):

R₁ = 510
R₂ = 1k
R₃ = 620
R₄ = 510

However, the input and feedback resistors affect the drive circuit, so the 150R resistor to gnd and the 2k2 resistor to +4V must be replaced with 50R and 150R respectively.

The solution above is implemented in two different circuit boards:

The custom MCU-board is considerably more complex than the shield and I’ve yet to confirm if there are any benefits (except for a direct USB-connection) over the simpler shield for the nucleo.

The custom board looks like this:

assembled-f405

Details can be found at the HaD project-page.

KiCAD project files and USB-firmware can be found in the downloads-section.


Suggested reading:
National’s Op Amp circuit collection
Analog’s data conversion handbook. Chapter 6
Bryant, Jung and Kester, Op Amp Applications, Chap. 3


Original posts at erossel.wordpress:
USB-firmware complete
Inverting and scaling
Replacing the STM32 Nucleo
An ideal world
STM32F4 SPL USB CDC VCP Loopback

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