LM9044 datasheet(9/14 Pages) TI

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LM9044

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SNOSBP4D – FEBRUARY 1995 – REVISED MARCH 2013

In this condition VOUT is:

VOUT = VREF x ((14k + 4k) / (26.5k + 14k + 4k))

VOUT = VREF x 0.4045

When VREF is at 5.0V, VOUT is defined as:

VOUT = 5.0V x 0.4045

VOUT = 2.0V

In the cases where both the inverting and non-inverting pins are open, the open inverting pin condition (i.e.: a

voltage divider across the output) will be the dominant condition.

Any common mode voltage transient on the inverting input pin which goes above the comparator threshold will

immediately cause the output to switch to the resistive voltage divider mode. The output will return to normal

operation when the voltage on the inverting input falls below the 1.5V threshold.

OUTPUT RESISTANCE

Under normal operating conditions the output pin resistance is typically 200

Ω.

If the LM9044 is operating in a default output mode due an open connection on the inverting input, the output

resistance will typically appear to be close to 11 k

Ω.

An external output filter capacitor value of no more than 0.01

μF is generally recommended. Since the output pin

voltage drive is basically a simple NPN emitter follower, the output pin pull-down is done by the internal feedback

resistor string. With larger value capacitors on the output pin the effect will be somewhat similar to a voltage peak

detector where the output capacitor is charged through the 200

Ω resistor, and discharged back through the 200Ω

resistor and the 18 k

Ω feedback resistor string to ground.

The output resistance provides current limiting for the output stage should it become shorted to Ground. Any DC

loading of the output will cause an error in the output voltage.

SUPPLY BYPASSING

For best performance the LM9044 requires a VREF supply which is stable and noise free. The same 5V reference

supply used for the A/D converter is the recommended LM9044 VREF supply.

The LM9044 VCC pin has an internal zener shunt voltage regulator, typically 7.5V, and requires a series resistor

to limit the current. The VCC pin should be bypassed with a minimum 0.01 μF capacitor to the Ground pin, and

should be located as close to the device as possible. Some applications may require an additional bypass

capacitance if the system voltage is unusually noisy.

SETTING THE BANDWIDTH

The LM9044 bandwidth is limited by an external capacitor (CF) on the RF pin.

This pin has an internal 175 k

Ω resistor. The external capacitor and the internal resistor form a simple RC low-

pass filter with a corner frequency (fC) defined as:

fC = 1/ (2 x π x 175 kΩ x CF)

With a CF capacitor value of 0.001 μF, the corner frequency is:

fC = 1/ (2 x π x 175 kΩ x 0.001 μF)

fC = 909 Hz

INPUT FILTERING

Filtering at the differential inputs is strongly recommended. Both the differential voltage signal and the common

mode voltage signal should have low pass filters.

Input filtering is accomplished with series resistors on the input pins, and appropriate bypass capacitors. Typical

input pin series resistance values are in the 100

Ω to 1kΩ range. Series resistance values larger than 1kΩ will

generate offset voltages that affect the accuracy of the signal voltage seen at the differential input pins.

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