# Low-Noise Programmable Voltage Source
## 2 Design of the Proposed Low-Noise Voltage Source  
The schematic diagram of the proposed voltage source is shown in Figure 1.  
The ATmega32 microcontroller (\(IC_{1}\)) supervising the operation of the system is controlled by a personal computer (PC) running a dedicated LabVIEW software via an insulated Universal Asynchronous Receiver-Transmitter (UART) interface to reduce interferences. For this purpose, we developed a simple board based on optocouplers (LTV817 by LITEON). The microcontroller drives the bipolar _DAC_ model AD5761R (\(IC_{2}\)) used to generate a programmable voltage. The DAC is supplied by lead acid batteries to reduce interferences from external environment. In order to insure low temperature drift and longtime stability at the DAC output, the low-noise voltage reference ADR441B (\(IC_{4}\)) was used. It is characterized by a temperature coefficient as low as 1 ppm/\({}^{\circ}\)C (max. of 3 ppm/\({}^{\circ}\)C). In order to obtain low noise, the DAC output voltage is filtered using two-stage lowpass RC filters separated by a low drift JFET input OPA140 operational amplifier (\(IC_{3}\)). The corner frequency of the first lowpass stage (\(R_{1}C_{1}\)) is 0.0023 Hz, while that of the second stage (\(R_{2}C_{2}\)) is 0.0016 Hz. Overall, the two cascaded lowpass filters provide an attenuation of the noise coming from the DAC of more  
Figure 1: Schematic diagram of the proposed low voltage source. than 60 dB at 0.001 Hz. The lowpass filters were built of film resistors and stable metallized polyester capacitors (WIMA MKS2). It must be noted that the resistances in the filers are sources of noise (thermal noise). However, at the output of an _RC_ filter and well above the corner frequency, the voltage noise \(V_{nRC}\) is given by:  
\[V_{nRC}=\sqrt{4 kTR\frac{f_{c}^{2}}{f^{2}}}, \tag{1}\]  
where \(f\) is the frequency, \(f_{c}=1/(2\pi RC)\) and \(k\) is the Boltzmann constant. With the values of the resistors and capacitors used in our design, this contribution becomes negligible above 0.001 Hz. In order to provide current to the load, the voltage at the output of the second RC filter is buffered by a low-noise source follower consisting of the selected four-paralleled 2 SK3557-6s or two paralleled IF9030 JFET transistors. Their drains were supplied by high-capacity NiMH batteries. To minimize the current noise generated by gate leakage current, that increases as the drain-to-gate voltage increases, a relatively low-supply voltage (\(V_{\text{B2}}\approx 6.5\) V) was used. Clearly, the DC voltage at the output of the system is not equal to the voltage at the output of the second filter because of the \(V_{GS}\) of the JFETs. The output voltage can however be monitored by the high-resolution (24-bit) analog-digital converter LTC2493 [33] (\(I\)\(C_{5}\)). In this way, the voltage of the DAC can be adjusted in order to obtain the desired output voltage. The \(S_{1}\) and \(S_{2}\) switches were used to speed up the charging/discharging process of filtering capacitors, which is particularly useful when setting any new output value. Finally, once steady state is obtained, the ADC input can be disconnected from the output in order to minimize interferences from the control system. The fact that the DAC can produce negative voltages makes it possible to obtain a 0 V output voltage. As far as the maximum output voltage is concerned, it mainly depends on the supply voltage \(V_{\text{B2}}\), the \(R_{\text{S}}\) resistor, and the value of the pinch-off voltage \(V_{\text{GS(off)}}\) of the JFET voltage of the used transistors. Note that, because the gate voltage (\(V_{\text{G}}\)) from DAC changes the bias point of the JFETs and hence the value of the their transconductance gain \(g_{m}\), the output resistance \(R_{OUT}\) of the source depends on the generated voltage since \(R_{OUT}=R_{\text{S}}\|\)(1/(\(g_{m}\))).