Jon Munson, Linear Technology
Design Note 513
Accurately measuring voltages requires minimizing the impact of the instrument connection to the tested circuit. Typical digital voltmeters (DVMs) use 10 M resistor networks to keep loading effects to an inconspicuous level, but even this can introduce significant error, particularly in higher voltage circuits that involve high resistances.
The solution is to use high impedance amplifiers in an electrometer configuration, so only miniscule amplifier input current comes from the test node. To make the input current the lowest possible value, field-effect transistors (FETs) are traditionally employed at the inputs of these circuits. FETs are generally low voltage devices, and introduce voltage offset uncertainty that is difficult to eliminate. Monolithic amplifiers exist that incorporate FET inputs, but these are often very low voltage devices, especially those using typical CMOS fabrication methods, so their utility is limited in high voltage applications. Enter the LTCR6090, a CMOS amplifier that can handle over 140 VP-P signal swings with sub-mV precision, ideally suited to tackle the problem.
The LTC6090 combines a unique set of characteristics in a single device. Its CMOS design characteristics provide the ultimate in high input impedance and “rail-to-rail” output swing, but unlike typical CMOS parts that might be powered by 5 V, the LTC6090 can operate with supplies up to ±70 V. The device can hold its own in the small-signal regime as well, featuring typical VOS under 500 μV and voltage-noise density of 11 nV/√Hz, yielding a spectacular dynamic range. With the high voltage operation comes the possibility of significant power dissipation, so the LTC6090 is available in thermally-enhanced SOIC or TSSOP packages. It includes an overtemperature output flag and an output disable control that provide flexible protection measures without additional circuitry.
The LTC6090 is capable of 140 V output levels in single-supply operation, so amplifying a quality 5 V reference is a simple matter of using accurate resistor networks to maintain precision. The LT5400 precision resistor array handles voltages up to 80 V, so utilizing the 10:1 ratio version for a gain of 10 is an easy way to produce an accurate 50 V calibration source without any adjustments required. Figure 1 shows a circuit amplifying the LT6654A 5.000 V reference to 50.00 V with better than 0.1% accuracy. The circuit may be powered from 55 V to 140 V, with 65 V being a useful supply voltage furnished from the optional portable supply shown as part of Figure 2.
|Figure 1.||High Voltage Precision Reference.|
The LTC6090 is set up with a 1 μF output capacitance to provide excellent load-step response. The capacitance is isolated from the op amp with a resistance that forms an effective noise-reduction filter for frequencies above 700 Hz. The precision LT5400A-3 resistor network provides 0.01% matched 10 k/100 k resistances that, along with the absence of loading by the high impedance CMOS op amp inputs, forms a highly accurate amplification factor. Input offset voltage of the LTC6090 contributes <0.03% error, while the LT6654A contributes <0.05%. The entire circuit of Figure 1 draws about 4 mA of quiescent current and can drive 10 mA loads.
The LTC6090 behaves as an ordinary unity-gain-stable operational amplifier, so constructing an electrometer-grade buffer stage is simply a matter of providing 100% feedback with the classic unity-gain circuit. No discrete FETs or floating biasing supplies are needed.
|Figure 2.||Buffered Probe for Digital Voltmeter.|
As shown in Figure 2, the LTC6090 can easily be powered with a split supply, such as a small flyback converting battery source. This basic circuit can provide precision measurement of voltages in high impedance circuitry and accurately handle signal swings to within 3 V of either supply rail (±62 V in this case). With input leakage current typically below 5 pA, circuit loading is essentially inconsequential (OS) for source impedances approaching a gigaohm. The useful full-swing frequency response is over 20 kHz.
The LTC6090 is a unique and versatile high voltage CMOS amplifier that enables simplified high impedance and/or large signal swing, very wide dynamic range amplification solutions.
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