LM331MWC【PDF 16/18 页】VOLTAGE-FREQUENCY CONVERTER, 0.1 MHz, UU

收藏本站

您好，
买卖IC网欢迎您。
请登录
免费注册

买卖IC网

Sheet目录39247

12页 13页 14页 15页 [16页]17页 18页

Typical Applications (Continued)

condition will usually apply under start-up conditions or in the

case of an overload voltage at signal input. It should be

noted that during this sort of overload, the output frequency

will be 0; as soon as the signal is restored to the working

range, the output frequency will be resumed.

The output driver transistor acts to saturate pin 3 with an ON

resistance of about 50

. In case of overvoltage, the output

current is actively limited to less than 50 mA.

The voltage at pin 2 is regulated at 1.90 V

DC for all values of

i between 10 A to 500 A. It can be used as a voltage ref-

erence for other components, but care must be taken to en-

sure that current is not taken from it which could reduce the

accuracy of the converter.

PRINCIPLES OF OPERATION OF BASIC VOLTAGE-

TO-FREQUENCY CONVERTER (

Figure 1)

The simple stand-alone V-to-F converter shown in

Figure 1

includes all the basic circuitry of

Figure 3 plus a few compo-

nents for improved performance.

A resistor, R

IN=100 k±10%, has been added in the path to

pin 7, so that the bias current at pin 7 (80 nA typical) will

cancel the effect of the bias current at pin 6 and help provide

minimum frequency offset.

The resistance R

S at pin 2 is made up of a 12 k fixed resis-

tor plusa5k

(cermet, preferably) gain adjust rheostat. The

function of this adjustment is to trim out the gain tolerance of

the LM231/331, and the tolerance of R

t,RL and Ct.

For best results, all the components should be stable

low-temperature-coefficient components, such as metal-film

resistors. The capacitor should have low dielectric absorp-

tion; depending on the temperature characteristics desired,

NPO ceramic, polystyrene, Teflon or polypropylene are best

suited.

A capacitor C

IN is added from pin 7 to ground to act as a filter

for V

IN. A value of 0.01 F to 0.1 F will be adequate in most

cases; however, in cases where better filtering is required, a

1 F capacitor can be used. When the RC time constants are

matched at pin 6 and pin 7, a voltage step at V

IN will cause

a step change in f

OUT.IfCIN is much less than CL, a step at

IN may cause fOUT to stop momentarily.

A47

resistor, in series with the 1 F C

L, is added to give

hysteresis effect which helps the input comparator provide

the excellent linearity (0.03% typical).

DETAIL OF OPERATION OF PRECISION V-TO-F

CONVERTER (

Figure 4)

In this circuit, integration is performed by using a conven-

tional operational amplifier and feedback capacitor, C

When the integrator’s output crosses the nominal threshold

level at pin 6 of the LM231/331, the timing cycle is initiated.

The average current fed into the op amp’s summing point

(pin 2) is i x (1.1 R

tCt) x f which is perfectly balanced with

IN/RIN. In this circuit, the voltage offset of the LM231/331

input comparator does not affect the offset or accuracy of the

V-to-F converter as it does in the stand-alone V-to-F con-

verter; nor does the LM231/331 bias current or offset cur-

rent. Instead, the offset voltage and offset current of the op-

erational amplifier are the only limits on how small the signal

can be accurately converted. Since op amps with voltage off-

set well below 1 mV and offset currents well below 2 nA are

available at low cost, this circuit is recommended for best ac-

curacy for small signals. This circuit also responds immedi-

ately to any change of input signal (which a stand-alone cir-

cuit does not) so that the output frequency will be an

accurate representation of V

IN, as quickly as 2 output pulses’

spacing can be measured.

In the precision mode, excellent linearity is obtained be-

cause the current source (pin 1) is always at ground potential

and that voltage does not vary with V

IN or fOUT. (In the

stand-alone V-to-F converter, a major cause of non-linearity

is the output impedance at pin 1 which causes i to change as

a function of V

IN).

The circuit of

Figure 5 operates in the same way as Figure 4,

but with the necessary changes for high speed operation.

www.national.com

发布紧急采购，3分钟左右您将得到回复。

采购需求

(若只采购一条型号，填写一行即可)

*型号	*数量	厂商	批号	封装

添加更多采购

采购需求

发布成功！您可以继续发布采购。也可以进入我的后台，查看报价

发布成功！您可以继续发布采购。也可以进入我的后台，查看报价

我的联系方式