QT301-D データシートの表示（PDF） - Quantum Research Group

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QT301-D

CAPACITANCE TO ANALOG CONVERTER

Quantum Research Group 

1 - Overview

The QT301 is a digital burst mode charge-transfer (QT)

capacitance-to-analog converter (CAC). It has a PWM output

designed for applications such as fluid level sensing and

distance gauging; the PWM signal is eight bits in resolution.

The IC features two calibration inputs for end-to-end span

calibration. The output depends on load (Cx) and sampling

capacitor (Cs) values.

1.1 Basic Operation

The QT301 has internal EEPROM to store the two calibration

points. The sensor acquires the signal from the electrode and

calculates the PWM result using the two calibration points.

The sensor can be calibrated via the two calibration inputs

(see Section 4). The signal can be acquired either

continuously or it can be synchronized on an external signal.

The response time of the PWM depends largely on the

acquisition burst spacing.

Table 1-1 Pin Description

Pin

Name

Function

1

SYNC

Sync Input

2

CAL_DN

Lower Calibration input

3

SNS1

Sense 1 line (to electrode)

4

VSS

Negative supply (ground)

5

SNS2

Sense 2 line

6

PWM

PWM output

7

CAL_UP

Upper Calibration input

8

VDD

Positive supply

2 - Signal Acquisition

The QT301 has a power-up delay of 200ms. During this

interval it does aquire signals or generate a PWM result; it

also ignores calibration inputs. This delay helps to prevent

false calibrations due to signal noise on Vdd during

startup.

Figure 1-1 Basic Circuit Diagram

Figure 2-1 shows the basic QT301 acquisition timing

parameters. Tbd is the burst duration, Tbs is the

3 to 5.5V

burst spacing from the start of one burst to the start

0.1uF

of the next burst; when there is no Sync signal Tbs =

Upper Cal

Lower Cal

8

R1

1 SYNC VDD

7 CAL_UP SNS1 3

2 CAL_DN SNS2 5

Rs

Cs

Tbd+2.5ms.

Electrode

Cx

2.1 Burst Properties

The QT301 employs bursts of charge-transfer cycles

to acquire its signal. Burst mode dramatically reduces

RF emissions and lowers susceptibility to EMI.

The acquisition burst operates in a band between

230kHz and 305khz. The burst is spread-spectrum

PWM Out

R3

6

PWM

VSS

R2

4

modulated within this band to suppress interference

from external noise sources.

The QT switches and charge measurement hardware

functions are all internal to the QT301. A 16-bit

single-slope switched capacitor, analog to digital

converter (ADC), includes both the required QT

charge and transfer switches in a configuration that

1.2 Basic Circuit

provides direct ADC conversion. The ADC is designed to

Figure 1-1 shows a basic circuit diagram for the QT301. The

pin layout of the QT301 is as explained in Table 1-1. In this

particular circuit, C1 should be 100nF and R1, R2 and R3

should all be 10K.

dynamically optimize the QT burst length according to the

rate of charge buildup on Cs, which in turn depends on the

values of Cs, Cx, and VDD. VDD is used as the charge

reference voltage.

R1 is only required if the synchronization feature is not used

and can be connected to either VDD or VSS.

Cs is recommended between 1nF and 500nF but this

depends on the sensitivity required. Use either NPO or PPS

capacitors for best results.

Figure 2-1 Acquisition Burst: No Sync Pulse

Rs is calculated with the following formula:

Rs

<

166%103

Cx

where Cx is expressed in pF.

Tbd

Tbs

LQ

2

QT301 R1.04 21/09/03

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