7

FN4367.2

January 6, 2006

Circuit Block Description

Input Amplifiers

Two amplifiers can be selected to interface to the engine

sensors. These amplifiers have a typical open loop gain of

100dB, with a typical bandwidth of 2.6MHz. The common

mode input voltage range extends to within 0.5V of either

supply rail. The amplifier output has a similar output range.

Sufficient gain, bandwidth and output swing capability is

provided to ensure that the amplifier can handle

attenuation gain settings of 20 to 1 or -26dB. This would be

needed when high peak output signals, in the range of

10 times can also be needed when the transducers have

In a typical application the input signal frequency may vary

from 1kHz to 20kHz. External capacitors are used to

decouple the IC from the sensor (C1 and C2) refer to

Figure 4. A typical value of the capacitor would be 3.3nF.

Series input resistors, R1 and R2, are used to connect the

inverting inputs of the amplifiers, (pins 19 and 16.)

Feedback resistors, R3 and R4, in conjunction with R3 and

R4 are used to set the gain of the amplifiers.

A mid voltage level is generated internally within the IC. This

Throughout the IC this level is used as a quiet, DC reference

for the signal processing circuits within the IC. This point is

brought out for several reasons, it can be used as a

reference voltage, and it must be bypassed to insure that it

remains as a quiet reference for the internal circuitry.

The input amplifiers are designed with power down

capability, which, when activated disables their bias circuit

and their output goes into a three-state state condition. This

is utilized during the diagnostic mode, in which the output

terminals of the amplifiers are driven by the outside world

with various test signals.

Antialiasing Filter

The IC has a 3rd order Butterworth filter with a 3dB point at

70kHz. Double poly capacitors and implanted resistors are

used to set poles in the filter. This filter is required to have no

more than 1dB attenuation at 20kHz (highest frequency off

interest) and a minimum attenuation of 10dB at 180kHz. This

filter precedes the switch capacitor filter stages which run at

the system frequency of 200kHz.

Programmable Gain Stage

The gains for two identical programmable gain stages can

be adjusted, so that the knock energies can be

compensated if needed. This adjustment can be made with

64 different gain settings, ranging between 2 and 0.111.

Programming is discussed in the SPI Communications

Protocol section.

Programmable Bandpass Filter

Two identical programmable filters are used to detect the

frequencies of interest. The Band Pass Filter (BPF) is

programmed to pass the frequency component of the engine

knock. The filter frequency is established by the characteristics

of the particular engine and transducer. By integrating the

rectified outputs from these two filters at the INTEGRATOR

stage, a knock can be detected if it has occurred.

The filters have a nominal differential gain of 4. Their

frequency is set by a programmable word (discussed in the

SPI Communications Protocol section.) Center frequencies

can be programed from 1.22kHz to 19.98kHz, in 64 steps.

The filter Qs are typically 2.4.

Active Full Wave Rectifier

The output of the bandpass filters are unity gain buffered prior

to full wave rectification using switch capacitor techniques.

Each side of the rectifier circuit provides both negative and

positive values of the knock frequency bandpass frequency

filter outputs. The output is able to swing from 20 to 80 percent

input to output of this stage.

Programmable Integrator Stage

The signals from the rectifier stage are separated into 2 output

signal paths which are then integrated together. A differential

system is used to minimize noise. One side integrates the

positive energy value from the Knock Frequency Rectifier.

The second side does the integration of the negative energy

value. The positive and negative energy signals are opposite

phase signals. Using this technique reduces system noise

from affecting the actual signal.

The integrator time constant is software programmable by

the Integrator Time Constant discussed in the

Communications Protocol section. The time constant can be

programmed from 40

µs to 600µs, with a total of 32 steps. If

for example, we program a time constant to 200

µs, then with

one volt difference between each channel, the output of the

integrator will change by volt in 200

µs.

SENSOR

R1

C1

R3

19

20

VMID

18

FIGURE 4. INPUT AMPLIFIER CONNECTIONS

-

+

SENSOR

R2

C2

R4

16

15

VMID

17

-

+

HIP9011