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Details, datasheet, quote on part number:QT114-D
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Datasheet text preview:
CHARGE-TRANSFER QLEVELTM SENSOR IC
! ! ! ! ! ! ! ! ! Limit sensing of almost any fluid or powder 2-Tier level sensor - Hi / Low limits with one probe Only one external part required - a 5¢ capacitor Uses internal probes or external electrodes Active high or active low outputs Slosh filter averages response of moving fluids LED drive capable on both outputs 2.5 to 5V 20µA single supply operation HeartBeatTM health indicator on both outputs
QProxTM QT114
Vcc Out1 Out2 Filt
1
8
Gnd Sns2 Sns1 Pol
Q T114
2 3 4
7 6 5
APPLICATIONS ! ! !
Process controls Vending machines Automotive fluids
! ! !
Consumer appliances Medical fluid sensing Soil moisture sensing
DESCRIPTION The QT114 QuickLevelTM charge-transfer ("QT") sensor IC is specifically designed to detect point level in fluids and powders. It will project a sense field through almost any dielectric, like glass, plastic, or ceramic, to sense lev el on the inside of a vessel, from its exterior. It has the unique capability of independently sensing two trip points when used with structured electrodes having two tiers. The QT114 does not have sensing timeouts, drift compensation, or other functions which would interfere with lev el sensing. Its threshold levels are fixed, and the amount of signal required to exceed a threshold is dependent on circuit gain and electrode size and loading, all of which are under the control of the designer. The QT114 requires only a single inexpensive capacitor in order to function. One or two LEDs can also be added to provide a visual sensing indication. Power consumption is under 20mA in most applications, allowing operation from Lithium cells for many years. In most cases the power supply needs only minimal regulation. The QT114 employs numerous signal acquisition and processing techniques pioneered by Quantum. No external switches, opamps, or other analog components aside from CS are required. A unique feature is the 'slosh filter', a detection integrator which averages detections over a rolling 15 second interv al before activating or deactivating the OUT pins. This filter allows use of the QT114 with violently moving fluids, for example in a moving vehicle, that would otherwise cause the outputs to flicker between two states. The device also includes selectable output polarity, allowing both output lines to be made either active-high or activ e-low. It also includes the Quantum-pioneered HeartBeatTM signal, allowing a host controller to monitor the health of the QT114 continuously if desired. By using the charge transfer principle, the IC delivers a level of perf ormance clearly superior to older technologies. It is specifically designed to replace electromechanical devices like float switches, thermistors, and conductance probes.
TA 00C to +700C -400C to +850C
AVAILABLE OPTIONS SOIC QT114-S QT114-IS
8-PIN DIP QT114-D
Copyright © 1999 Quantum Research Group Ltd R1.03
Quantum Research Group Ltd
The QT114 is a digital burst mode charge-transfer (QT) sensor designed specifically for point level sensing; it includes all hardware and signal processing functions necessary to provide stable level sensing under a wide variety of changing conditions. Only a single external capacitor is required for operation. Figure 1-1 shows a basic QT114 circuit using the device, with conventional OUT drives and power supply connections. The sensing electrode can be connected to a single-tier or 2-tier electrode as required. Calibration is done by design, through adjustment of the electrode sizes and the Cs capacitor. Only under rare situations do QT114 circuits require calibration on an individual basis, and the circuit can make provision for that.
To 10x Scope Probe Vcc 1 O UT 1 O UT 2 2 OUT1 SNS2 7 3 OUT2 SNS1 6 4 FILT POL 5 8 Gn d Vdd FILTER POLARITY POL : 1 = Active High FILT: 1 = Slosh Filter Vdd 2M (optional)
Cs
To Electrode(s)
1M multi-turn pot (optional)
1 - SIGNAL ACQUISITION
The QT114 employs a short, low duty cycle burst of charge-transfer cycles to acquire its signal. Burst mode perm its power consumption in the low microamp range, dram atically reduces RF emissions, lowers susceptibility to EMI, and yet permits excellent response time. Internally the signals are digitally processed to generate the required output signals. The QT switches and charge measurement hardware functions are all internal to the QT114 (Figure 1-2). A 14-bit single-slope switched capacitor ADC includes both the required QT charge and transfer switches in a configuration that provides direct ADC conversion. The burst length is inversely proportional to the rate of charge buildup on Cs, which in turn depends on the values of Cs, Cx, and Vcc. Vcc is used as the charge reference voltage. Larger values of Cx cause the charge transferred into Cs to accumulate more rapidly. The trip points of the sensor can be changed by altering Cs and Cx, the load capacitance. As a result, the values of Cs, Cx, and Vcc should be fairly stable over the expected operating temperature range. Two fixed thresholds are used, one for low fluid level and the other for high level; adjusting Cs and Cx to allow these to trip at appropriate points is required by design, and if required m ay be trimmed by an adjustment. Figure 1-1 shows the optional potentiometer which can be used to fine-tune the placement of these threshold points relative to the signal.
Figure 1-1 Standard mode options It is not necessary to use both detection threshold points; if only single point sensing is desired, only the lower threshold and OUT1 can be used, while ignoring OUT2. Two option pins allow the selection of output polarity and the insertion of a 'slosh filter' before the OUT pins, as shown in Figure 1-1.
1.1 ELECTRODE DRIVE
The internal ADC treats Cs as a floating transfer capacitor; as a direct result, the sense electrode can be connected to either SNS1 or SNS2 with no performance difference. The polarity of the charge buildup across Cs during a burst is the sam e in either case. Cs must be of within a certain range for proper operation. It is possible to connect separate Cx and Cx' loads to SNS1 and SNS2 simultaneously, although the result is no different than if the loads were connected together at SNS2 (or SNS1). It is important to limit the amount of stray capacitance on both terminals, especially if the load Cx is already large, for example by minimizing trace lengths and widths so as not to exceed the Cx load specification and to allow for a larger sensing electrode size if so desired. The PCB traces, wiring, and any components associated with or in contact with SNS1 and SNS2 will become proximity sensitive and should be treated with caution.
1.2 THRESHOLD POINTS
E LE C T R O D E R e su l t S ing le -S lo p e 14-bit S w i tch e d Capacitor ADC
S NS 2
Bu rst Controller
Cs Cx
S NS 1
The QT114 employs twin threshold points set at both 250 (for T1) and 150 counts (for T2) of acquisition signal. The signal travels in an inverse direction: increasing amounts of Cx reduce the signal level; the baseline ('dry') signal should lie at 300 counts or m ore under most conditions. Calibration details are discussed fully in Section 3.2.
S ta r t
Do ne
2 ELECTRODE DESIGN
The QT114 is designed to operate with a 'plateau' sensor, having a substantial surface area at each desired trip point, to create a capacitive 'step'. As Figure 2-1 shows, a vertical strip sensor on the outside of a container (or a vertical, insulated rod in the fluid) will generate a long sloping signal. The desired trip point 'T' is subject to a great deal of variation in location if the sensing signal drifts much,
C ha r g e Amp
Figure 1-2 Internal Switching & Timing
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for example due to changes in Cs or Cx over the operating temperature range. Figure 2-2 shows the response from a horizontal strip of the same surface area; the signal exhibits a very rapid rise in signal between points l1 and l2. Variations in circuit gain or signal drift have much less of an effect on the trip point with this orientation. In some cases (thin walled vessels for example) it may be sufficient to have a small round or square electrode patch on the exterior. Figure 2-3 shows the response from a twin-level external electrode set. The use of two horizontal electrode planes or tiers creates well-defined trip points that can be used to sense both 'low' and 'high' levels. A crossing of threshold T1 will be reflected in the OUT1 signal, while T2 will be reflected on OUT2.
S ign a l
l2 T1 l1 T1 Le ve l
l1
l2
Figure 2-1 Signal vs. Level for an External Vertical Strip
S ig n a l
2.1 EXTERNAL ELECTRODES
External electrodes should be electrically conductive; m etal foils and conductive carbon are both possible. Care should be taken that other objects or people near the vessel will not touch the electrode; in some cases shielding around the electrode with grounded metal will be required to prevent disturbances. If used, the shield elem ent should be spaced apart from the electrode by an air gap or a low-density foam to reduce Cx loading. The required surface area of the external electrode will depend on the amount of signal needed to bracket the detection threshold, which in turn will depend in part on Cs and stray Cx. External electrodes sensing through thick walls and/or sensing low permittivity fluids will require larger surface areas than those sensing water through thin plastic, for example. External electrodes are more likely to require potentiometer trimming to achieve reliable operation (Figure 1-1, also Section 3.2).
l1
l2
T1 T1 Le v e l
l1 l2
Figure 2-2 Signal vs. Level for an External Horizontal Strip
S i gn a l
T2 l3 l4 T2
Note that external electrodes used with conductive solutions (i.e. aqueous liquids) do not measure the T1 l T1 perm ittivity of the fluid: they actually measure the l1 2 perm ittivity of the vessel wall, between 2 plates: the Le v e l electrode (plate 1) and the fluid (plate 2, effectively a l 3 l4 l1 l2 variable-area ground plate): if the fluid were to be replaced with mercury the signal would be unchanged. A 20% Figure 2-3 Signal vs. Level for Twin Horizontal Strips thickness variation in the vessel wall will therefore introduce about a 20% variation in the resulting capacitance; if the vessel wall cannot be controlled 2.2.1 DISC PROBES accurately enough in production, serious sensing errors may The simplest internal geometry is probably a disc probe (Figure 2-4), having at least one planar surface ('tier') occur. parallel to the fluid surface. The sensing error can be W hen external electrodes are used to sense non-aqueous m inim ized by making the tier thin, so that the signal substances (like oils or gasoline), the vessel wall dielectric transitions abruptly higher (see Figure 2-2) as the fluid becom es a lessor contributor to the overall signal, which is covers the tier. then heavily dominated by the permittivity of the fluid. The A notable difficulty with disc probes is the task of insulating lower the permittivity of the fluid the greater its dominance. them with a uniform, repeatable thickness of insulation.
2.2 INTERNAL PROBES
W hen used with aqueous fluids or other electrically conducting liquids, internal probes should be insulated with a plastic layer. See also Section 2.1 for a discussion of electrodes when used with conductive fluids. Aqueous probes should be 100% insulated, even on the cut end of a wire probe. The slightest pinhole of exposed metal anywhere on an immersed part of the probe will immediately convert the probe into a bare-metal probe (see Section 2.2.5). Numerous types of internal point-level probes are possible.
2.2.2 SPIRAL WIRE PROBES
A spiral solid-wire probe is simple to construct (Figure 2-5), and has the advantage of being pre-insulated in a wide choice of plastics from inexpensive PVC to PTFE. These probe types provide a large step-function of capacitance localized at the desired trip point, and are easy to form. Spiral wire probes are most effective in water-based fluids; they are not as effective in oils and other nonconductive substances.
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