4-6

Description of the HIP9010 Operation

This IC is designed to be a universal digitally controlled,

analog interface between engine acoustical sensors or

accelerometers and internal combustion engine fuel

management systems. Two wideband input amplifiers are

provided that allow the use of two sensors that may be of the

piezoelectric type that can be mounted in optimum locations

on either in-line or V-type engine configurations.

Output from these amplifiers is directed from a channel

select switch into both digitally controlled filter and amplifier

channels. Both filter bandpass and gain settings are

programmable from a microprocessor. Output from the two

channels is combined in a digitally programmable integrator.

Integrator output is applied to a line driver for further

processing by the engine fuel management system.

Broadband piezoelectric ceramic transducers used for the

engine signal pickup have device capacitances in the order

of 1100pF and output voltages that range from 5mV to

channel is selected and applied to the filters. One filter

channel processes a signal that is used to establish the

background reference level. The second channel is used to

observe the engine during the time interval that preignition

may be expected. This information is compared with the

“background” signal via the IC’s integrator and will tend to

cancel the background noise and accentuate noise due to

engine pre-detonation. Moreover, the bandpass of filter

channels can be optimized to further discriminate between

engine background and combustion noise and

pre-detonation noise.

A basic approach to engine pre-detonation systems is to only

observe engine background during the time interval that noise

is expected and if detected, retard timing. This approach does

not require the sensitivity and selectivity that is needed for a

continuously adjustable solution. Enhanced fuel economy and

performance is obtainable when this IC is coupled with a

microprocessor controlled fuel management system.

Circuit Block Description

Input Amplifiers

Two amplifiers are used to interface to the two 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 was

provided to ensure that the amplifiers can handle attenuation

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

obtained from the transducer. Gain settings of 10 times can

also be needed when the transducers have output levels of

In a typical application the input signal frequency may vary

from DC to 20kHz. External capacitors are used to decouple

the IC from the sensor (C1 and C2). A typical value of the

capacitors is 3.3nF. Series input resistors, R1 and R2, are

used to connect the inverting inputs of the amplifiers, (pins

20 and 17). Feedback resistors, R3 and R4, in conjunction

with R1 and R2 are used to set the gain of the amplifiers.

A mid-voltage level is generated by the IC. This level is set to

this level is used as a quiet, DC reference for the 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 ensure that it is 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 condition. This is very important

during the test mode, in which the output terminals of the

amplifiers are driven by the outside world with test signals.

Antialiasing Filter

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

70kHz. Double poly-silicon 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

filters which run at 200kHz.

Programmable Band Pass Switched Capacitor Filters

Two identical programmable filters are used to detect the two

frequencies of interest. The Knock Frequency Filter is

programmed to pass the frequency component of the engine

knock. The Reference Frequency Filter is used to detect

background noise at a second programmed frequency. The filter

frequency is established by the characteristics of the particular

engine and transducer. By subtracting the energy component of

these two filters, we can detect if a knock has occurred.

The filters have a nominal differential gain of 4. Their

frequency is set by program words (discussed in the

Communications Protocol section). Center frequencies can

be programmed from 1.22kHz to 19.98kHz, in 64 steps. The

filter Qs are typically 2.4.

0

SENSOR

C1

R1

R3

PIN 20

PIN 3

-

+

PIN 19

1

SENSOR

C2

R2

R4

PIN 17

PIN 3

-

+

PIN 18

FIGURE 3. INPUT AMPLIFIER CONNECTIONS

HIP9010