DESIGN FEATURES LT1880 SOT-23 Op Amp Saves Board Space in Precision Applications by Glen Brisebois Introduction The tiny new LT1880 achieves precision unprecedented in a SOT-23 package without resorting to autozeroing techniques. Input offset voltage and drift are typically 40µV and 0.3µV/°C, respectively, with guarantees of 200µV and 1.2µV/°C maximum over temperature. The device operates on total supplies from 2.7V to 40V with rail-to-rail outputs, giving a dynamic range of 120dB. Unlike some competitors’ SOT-23 op amps, which claim to maintain good precision, the LT1880 supports its input precision with a high open loop gain of 1.6 million, as well as 135dB CMRR and PSRR. It is available in commercial and industrial temperature grades. The circuit of Figure 2 shows an extreme example of the inverting case. The input voltage at the 1M resistor can swing ±13.5V and the LT1880 will output an inverted, divided-byten version of the input voltage. The input accuracy is limited by the resistors to 0.2%. Output referred, this error becomes 2.7mV. The 40µV input offset voltage contribution, plus the additional error due to input bias current times the ~100k effective source impedance, contribute only negligibly to error. + RG VIN + VIN LT1880 – VIN 1M, 0.1% 100k, 0.1% –1.5V Figure 2. Extreme inverting case: circuit operates properly with input voltage swing well outside op amp supply rails. CF VIN + WORST-CASE OUTPUT OFFSET ≤196µV AT 25°C ≤262µV 0°C TO 70°C ≤323µV –40°C TO 85°C RF 51.1k 5V – PHOTODIODE (SEE TEXT) CD LT1880 OUT + –5V Figure 3. Precision photodiode amplifier mally flat response and highest possible bandwidth given a photodiode capacitance CD. Figure 4 shows a chart of CF and rise time versus CD for maximally flat response. Total output offset is below 262µV, worst-case, over temperature (0°C–70°C). With a 5V output swing this implies an 86dB 100 – – – RF RF RG VREF INVERTING: AV = –RF/RG OP AMP INPUTS DO NOT MOVE, BUT ARE FIXED AT DC BIAS POINT VREF INPUT DOES NOT HAVE TO BE RAIL-TO-RAIL NONINVERTING: AV = 1 + RF/RG INPUTS MOVE BY AS MUCH AS VIN, BUT THE OUTPUT MOVES MORE INPUT MAY NOT HAVE TO BE RAIL-TO-RAIL NONINVERTING: AV = +1 INPUTS MOVE AS MUCH AS OUTPUT INPUT MUST BE RAIL-TORAIL FOR OVERALL CIRCUIT RAIL-TO-RAIL PERFORMANCE Figure 1. Some op amp configurations do not require rail-to-rail inputs to achieve rail-to-rail outputs. Linear Technology Magazine • May 2001 RISE TIME (µs), CF (pF) VREF + Precision Photodiode Amplifier Photodiode amplifiers usually employ JFET op amps because of their low bias current; however, when precision is required, JFET op amps are Applications generally inadequate, due to their relatively high input offset voltage Getting Rail-to-Rail Operation and drift. The LT1880 provides a high without Rail-to-Rail Inputs degree of precision with very low bias The LT1880 does not have rail-to-rail current (IB = 150pA typical) and is inputs, but for most inverting appli- therefore applicable to this demandcations and noninverting gain ing task. Figure 3 shows an LT1880 applications, this is largely inconse- configured as a transimpedance phoquential. Figure 1 shows the basic op todiode amplifier. The transimpedance amp configurations, what happens to gain is set to 51.1kΩ by RF. The feedthe op amp inputs, and whether or back capacitor, CF, may be as large as not the op amp must have rail-to-rail desired where response time is not an inputs. issue, or it may be selected for maxi- ±1.35V OUTPUT SWING 1.5V ±13.5V SWINGS WELL OUTSIDE SUPPLY RAILS CF 10 RISE TIME 1 100mV OUTPUT STEP 0.1 0.1 1 10 CD (pF) 100 1000 Figure 4. Feedback CF and rise time vs photodiode CD 23 DESIGN FEATURES 18dB by the higher value set resistor. The output impedance is much lower, being determined by the output impedance of the LT1880. The circuit of Figure 7 achieves essentially the same function, but is stabilized into a 10µF reservoir capacitor. LT1634ACS8-4.096 –4.096V OUT R1 32.4k –5V Figure 5. Simple, precision –4.096V reference: output is not buffered. PSRR is compromised by low value of R1. Single-Supply Current Source for Platinum RTD dynamic range, worst-case, sustained over temperature (0°C–70°C), and a full-scale photodiode current of 98µA. Invert, Buffer Your Shunt Reference and Improve PSRR Figure 5 shows a negative 4.096V precision LT1634 shunt reference operating on a –5V supply. R1 is set at 32.4k to give 20µA of bias current to the reference with a minimum negative supply of –4.75V. Although this is an excellent way to achieve a precision negative reference, the circuit can be compromised in some applications in two ways. One is by its high, large signal output impedance and the other is by its 70dB PSRR dictated by the 32.4k resistor from the negative supply and the ~10Ω dynamic impedance of the LT1634 at 20µA bias. The circuit in Figure 6 uses the LT1880 to solve both problems. The op amp is configured as an inverter with the LT1634 in the feedback path. The bias current of the LT1634 is thus maintained at a constant level by the 249k resistor and the LT1880. The power supply rejection, now positive-supply referred, is improved by 5V 249k – + LT1634ACS8 -4.096 The precision, low bias current input stage of the LT1880 makes it ideal for precision integrators and current sources. Figure 8 shows the LT1880 providing a simple precision current source for a remote 1kΩ RTD on a 4-wire connection. The LT1634 reference places 1.25V at the noninverting input of the LT1880, which then maintains its inverting input at the same voltage by driving 1mA of current through the RTD and the total 1.25kΩ of resistance set by R1 and R2. Imprecise components R4 and C1 ensure circuit stability, which would otherwise be excessively dependant on the cable characteristics. R5 is also noncritical and is included to improve ESD immunity and decouple any cable capacitance from the LT1880’s output. The 4-wire cable allows Kelvin sensing of the RTD voltage while excluding the cable IR drops from the voltage reading. With 1mA excitation, a 1kΩ RTD will have 1V across it at 0°C, and +3.85mV/°C temperature 5V LT1880 –5V –4.096V OUT Figure 6. Precision –4.096V reference: LT1880 buffers the output voltage, improves PSRR. response. This voltage can be easily read in myriad ways, with the best method depending on the temperature region to be emphasized and the particular ADC that will be reading the voltage. Conclusion The precision, low bias current input stage of the LT1880 makes it ideal for precision and high impedance circuits. The rail-to-rail output stage renders the op amp capable of driving other devices as simply as possible with extended dynamic range, while the 2.7V to 40V operation means that it will work on almost all supplies. The small SOT-23 package makes it a compelling choice where board space is at a premium, or where a composite amplifier is competing against a larger single-chip solution. R5 180Ω, 5% + R1 249k R2 10k R4 1k, 5% 5V R1 1.24K 0.1% – + LT1634ACS8 -4.096 1kΩ AT 0°C RTD* 0.1µF 5V VOUT = 1.00V AT 0°C + 3.85mV/°C – –50°C TO 600°C R2 10Ω 1% LT1880 R3 47Ω C1 0.1µF 5V – + LT1880 R3 150k, 1% + 10µF LT1634ACS8 5V -1.25 –5V –4.096V OUT *OMEGA F3141 1kΩ, 0.1% PLATINUM RTD Figure 7. Same as Figure 6 except the circuit is stabilized into a large capacitance. 24 (800) 826-6342 Figure 8. Single-supply current source for platinum RTD Linear Technology Magazine • May 2001