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Part: 4127KG
Category:
Analog & Mixed-Signal Processing
-> Amplifiers
-> Logarithmic Amplifiers
Description: ti 4127, Logarithmic Amplifier
Company: Texas Instruments, Inc.
Datasheet: Download 4127KG datasheet File size : 103 kB
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®
4127
LOGARITHMIC AMPLIFIER
FEATURES
q ACCEPTS INPUT VOLTAGES OR CURRENTS OF EITHER POLARITY q WIDE INPUT DYNAMIC RANGE 6 Decades of Current 4 Decades of Voltage q VERSATILE Log, Antilog, and Log Ratio Capability current and four decades of input voltage. In addition, a current inverter and a precise internal reference allow pin programming of the 4127 as a logarithmic, log ratio, or antilog amplifier. To further increase its versatility and reduce your system cost, the 4127 has an uncommitted operational amplifier in its package that can be used as a buffer, inverter, filter, or gain element. The 4127 is available with initial accuracies (log conformity) of 0.5% and 1.0%, and operates over an ambient temperature range of 10°C to +70°C. With its versatility and high performance, the 4127 has many applications in signal compression, transd u c e r linearization, and phototube buffering. Manufacturers of medical equipment, analytical instruments, and process control instrumentation will find the 4127 a low cost solution to many signal processing problems.
DESCRIPTION
Packaged in a ceramic double wide DIP, the 4127 is the first hybrid logarithmic amplifier that accepts signals of either polarity from current or voltage sources. A special purpose monolithic chip, develo p e d specifically for logarithmic conversions, functions accurately for up to six decades of input
+15VDC 15VDC
International Airport Industrial Park · Mailing Address: PO Box 11400 Tel: (520) 746-1111 · Twx: 910-952-1111 · Cable: BBRCORP ·
©
· Tucson, AZ 85734 · Street Address: 6730 S. Tucson Blvd. · Tucson, AZ 85706 Telex: 066-6491 · FAX: (520) 889-1510 · Immediate Product Info: (800) 548-6132 PDS-346F Printed in U.S.A. October, 1993
1976 Burr-Brown Corporation
SBFS010
SPECIFICATIONS
ELECTRICAL
Typical Specifications at +25°C with rated supplies, unless otherwise noted. MODEL ACCURACY(1), % of FSR Current Source Input: 1nA to 1mA Voltage Input: 1mV to 10V INPUT Current Source Input, Pin 4 Current Source Input, Pin 7 Reference Current Input, Pin 2 Absolute Maximum Inputs OUTPUT Voltage Current Impedance FREQUENCY RESPONSE 3dB Small Signal at Current Input of 100µA of 10µA of 1µA of 100nA of 10nA Step Response to within ±1% of Final Value (IR = 1µA, A = 5) STABILITY Scale Factor Drift (A/°C) Reference Current Drift (IR/°C) Input Offset Current Drift (IS/°C) Input Offset Voltage Drift Accuracy vs Supply Variation Reference Current Input Offset Voltage Input Noise - Current Input Input Noise - Voltage Input 4127KG 0.5% max 0.5% max 4127JG 1% max 1% max
PIN CONFIGURATION
Top View
I REF
Output Input
2 1 3 4 5 6 7 8 9
24 No Pin Present 23 P EFBias IR 22 ostive Supply
REF
No Pin Present C +I Input
(1) (1)
+1nA to +1mA 1nA to 1mA +1µA to +1mA ±10mA or ±Supply Volts ±10V ± 5mA 10
21 Common N 10 o Pin Present
N urrent Inverter Output
o Pin Present Current Inverter Input No Pin Present
19 Gain Adjust 18 Log Output 17 No Pin Present 16 No Pin Present 15 No Pin Present 14 Negative Supply 23 NC
Op Amp + Input 1
90kHz 50kHz 5kHz 250Hz 80Hz 10ms ±0.0005A/°C ±0.001 I R/°C for IR 1µA ±0.003 IR/°C for 400nA < IR < 1µA 10pA at +25°C, Doubles Every 10°C ±10µV/°C ±0.001IR/V ±300µV/V 1pA, rms, 10Hz to 10kHz 10µA, rms, 10Hz to 10kHz 5mV 40nA 1M 85dB 5mA
Op Amp Input 10 Op Amp Output 11 N No IPin Present 2
OTE: (1) Pins 4 and 5 are internally connected.
PACKAGE INFORMATION
MODEL 4127KG 4127JG PACKAGE 24-Pin 24-Pin PACKAGE DRAWING NUMBER(1) 075 075
UNCOMMITTED OP AMP CHARACTERISTICS Input Offset Voltage Input Bias Current Input Impedance Large Signal Voltage Gain Output Current TEMPERATURE RANGE Specification Operating Storage POWER SUPPLY REQUIREMENTS Rated Supply Voltages Supply Voltage Range Supply Current Drain at Quiescent, max at Full Load, max NOTE: (1) Log conformity at 25°C.
NOTE: (1) For detailed drawing and dimension table, please see end of data sheet, or Appendix D of Burr-Brown IC Data Book.
0°C to +60°C 10°C to +70°C 55°C to +125°C ±15VDC ±14VDC to ±16VDC ±20mA ±26mA
The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes no responsibility for the use of this information, and all use of such information shall be entirely at the user's own risk. Prices and specifications are subject to change without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant any BURR-BROWN product for use in life support devices and/or systems.
®
4127
2
TYPICAL PERFORMANCE CURVES
At +25°C with rated supplies, unless otherwise noted.
A RELATIONSHIP OF REFERENCE CURRENT, IR ND EXTERNAL RESISTOR, R1 100µA 1 10 9 8 7 Scale Factor "A" (V) 1µA 0
RELATIONSHIP OF SCALE FACTOR "A" T O GAIN SETTING RESISTOR, R2
Reference Current
6 5 4 3 2 1 EO = A log10 0 |IS| IR 80
1
µA
00nA EO = A log10 10nA 10k 100k 1M Resistance () 10M 100M IS IR
0
20
40 Resistance (k)
60
LOG RELATIONSHIP OF
AND OUTPUT IR VOLTAGE IN TERMS OF "A" |IS| IR
|IS|
RELATIONSHIP OF
AND OUTPUT VOLTAGE IR For IR = 1µA and A = 5V and 10V EO = A log A = 10V C Input urrent IS
1µA 10µA 100µA
|IS|
V Output oltage 4A EO Volts
EO = A log10
V Output oltage 10V 5V A = 5V
0.01µA 0.1µA
|IS| IR
2A
10 I
R
C Input urrent IS
1I00 1000 I
R R
0.I 01 0I01 0.1 0 . I
R R R
IR
2A
5V 10V
40dB 80dB
Range of Adjustment
4A
DISCUSSION OF SPECIFICATIONS
ACCURACY The deviation from the ideal output voltage defined as a percent of the full scale output voltage. INPUT/OUTPUT RANGE The log relationships of A log STABILITY The use of a monolithic transistor quad and low-drift amps minimizes drift, but some drift remains in the scale-factor, reference current, and input offset. Input offset consists of a bias current plus the op amp input voltage offset divided by the signal source resistance. Also, there is some slight drift in conformity to the log function and in output amplifier offset, but this is generally negligible.
IS IR
and A log
ES IRR
are
subject to the constraints specified. The 4127 can be operated with inputs lower than those given, but the accuracy will be degraded. FREQUENCY RESPONSE The small-signal frequency response varies considerably with signal level and scaling, so the frequency response is specified under several different operating conditions.
THEORY OF OPERATION
The 4127 is a complete logarithmic amplifier that can be pin-programmed to accept input currents or voltages of either polarity. By making use of the internal current inverter, reference current generator, log ratio element, and uncommitted op amp, you can generate a variety of logarith®
3
4127
mic functions, including the log ratio of two signals, the logarithm of an input signal, or the antilog of an input signal. The unique FET-input current-inverting element removes the polarity limitations present in most conventional log amplifiers. Utilizing the inherent exponential characteristics of transistor functions, the 4127 calculates accurate log functions for input currents from 1nA to 1mA, or input voltages from 1mV to 10V. Carefully matched monolithic quad transistors and temperature sensitive gain elements are used to produce a log amplifier with excellent temperature characteristics. A functional diagram of the 4127 circuit is shown in Figure 1. In addition to the basic log amplifier, the 4127 contains a separate internal current source, a current inverter, and an uncommitted operational amplifier. The current inverter accurately converts negative input current to a positive current of equal magnitude. The 4127 is capable of accurately logging input current over a 120dB range, but to use this full range, good shielding practice must be followed. A current source input is, by definition, a high impedance source and is therefore subject to electrostatic pickup. The input op amps, A1 and A3, have FET input stages for low noise and very-low input bias current. The op amp, A1, will make the collector current of Q1 equal to the signal input current IS, and the collector current of Q2 will be the reference input current IR.
From the semiconductor junction characteristics, the baseto-emitter voltage will be: VBE mKT ln q IC IL ,
where: IC = Collector current IL = Reverse saturation current q, m, K = Constants T = Absolute temperature So E1 = IS IR mKT1 mKT2 ln and E2 E1 = ln q IL1 q I L2
If the transistors Q1 and Q2 are at the same temperature and have matched characteristics, then: E2 = mKT q
ln
IR IL IS IR
ln
IS IL
E2 =
mKT ln q
The output op amp, A2, provides a voltage gain of approximately (RT + R2)/RT, and the value of (mKT)/q is about 26mV at room temperature. Since resistor RT varies with temperature to compensate for gain drift, the output voltage, EO, expressed as a log will be:
I urrent C nverter I nput 7 I urrent C Overter n utput 5
T
IS A3 IS
Q
520 Thermistor
19
A Gain djust R G2
2
R2 E
A2
EO
18
ain Log Output
10 Q1
Op Amp Input
+IINPUT I S 4 A1
IIREF nput IR
A4 E1
11 Op Amp Output 9 p Amp +Input
2 IR 1 I O REF utput 15VDC
5k
23
REF O
IR R
1
22 I +15VDC 14
+15VDC 15VDC
21
Common
FIGURE 1. Functional Diagram.
®
4127
4
EO = A log10 RT + R2 RT
IS IR 1
,
IMAX IR , RT 520 log IMAX IR
=
104 = 100 106
where A
(26mV)
= 2; So, A = 5
0.434
The external resistor R1 sets the reference current IR and resistor R2 sets the scale-factor "A". R1 and R2 must be trimmed to the desired values, but the approximate relationships are shown in Typical Performance Curves. The relationship between the input current, IS, and the output voltage, EO, in terms of the externally adjusted parameters, IR and "A", is illustrated in Typical Performance Curves. This relationship is, of course, restricted to values of IS between 1nA and 1mA and output voltages of less than ±10V. CHOOSING THE OPTIMUM SCALE FACTOR AND REFERENCE CURRENT To minimize the effects of output offset and noise, it is usually best to use the full ±10V output range. Once an output range of ±10V has been chosen, then "A" and IR can be determined from the Min/Max of the input current, IS. EO = A log IS IR , where IMIN < IS < IMAX
For an IR of 1µA and A of 5, EO = 5log IS 1µA
CONNECTION DIAGRAMS
Transfer function is EO = A log I1 IR where I1 is a positive input current and IR is the resistor-programmed internal reference current (see Figure 2).
15VDC Cference e urrent I 15VDC
+
2 1 3 2
N
21
14 8 1 19 2
O
4127 24
R 1 (1) 3
Gain R
E 2
The output range of ±10V for an input range of IMIN to IMAX means that:
+15VDC
OTE: (1) Needed only if I1< 10nA. 1 4(1) R 0kR 15VDC
+10 = A log
IMIN IR
and 10 = A log
IMAX IR
Adding these two equations together log IMAX + IMIN IR2 = 0, or IR = IMAX IMIN
FIGURE 2. Transfer Function When I1 is Positive. ADJUSTMENT PROCEDURE 1. Refer to Choosing the Optimum Scale Factor and Reference Current. 2. Apply |I1| = IR, adjust R1 such that EO = 0. 3. Apply |I1| = IMAX, adjust R2 for the proper output voltage. 4. Repeat steps 2 and 3 if necessary. 5. Ignore this step if |I1MIN| 10nA. Otherwise, apply |I1| = 1nA, make R3 = 1kM and adjust R4 for the proper output voltage. For R3, a single resistor is recommended. A voltage divider network is difficult to use due to amplifier offset voltage. Transfer function is EO = A log |I1| IR where I1 is a negative
The value for A can be found from: 10 = A log IMAX IMAX IMIN In terms of the input current range for IS, the values for IR and A that will provide a full ±10V output swing are: IR = IMAX IMIN and A = log EXAMPLE Assume that IMIN is +10nA and IMAX is +100µA. This is an 80dB range. IR = IMAX IMIN = (104) (108) = 106, or 1µA. 10 IMAX IR
input current and IR is the resistor-programmed internal reference current (see Figure 3).
®
5
4127
Others parts begin by 41
41-1
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