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.

1.2 Basic Circuit

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.

R1 is only required if the synchronization feature is not used

and can be connected to either VDD or VSS.

Cs should be between 1nF and 500nF depending on the

sensitivity required. Use either NPO or PPS capacitors for

best results.

Rs is calculated with the following formula:

3

Cx

where Cx is expressed in pF.

2 - Signal Acquisition

The QT301 has a power-up delay of 300ms. During this

interval it does not acquire 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 2-1 shows the basic QT301 acquisition timing

parameters. Tbd is the burst duration, Tbs is the

burst spacing from the start of one burst to the start

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

Tbd+2.5ms.

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 310kHz. The burst is spread-spectrum

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

provides direct ADC conversion. The ADC is designed to

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.

2

QT301 R1.06 12/03

Positive supply

VDD

8

Upper Calibration input

CAL_UP

7

PWM output

PWM

6

Sense 2 line

SNS2

5

Negative supply (ground)

VSS

4

Sense 1 line (to electrode)

SNS1

3

Lower Calibration input

CAL_DN

2

Sync Input

SYNC

1

Function

Name

Pin

Table 1-1 Pin Description

Figure 1-1 Basic Circuit Diagram

7

1

2

8

6

CAL_UP

PWM

SNS2

SNS1

VDD

5

3

SYNC

VSS

3 to 5.5V

Electrode

Cs

Cx

CAL_DN

Rs

0.1uF

4

R1

R2

R3

Upper Cal

Lower Cal

PWM Out

Figure 2-1 Acquisition Burst: No Sync Pulse

Tbd

Tbs