DESIGN FEATURES SOT-23 Digitally Controlled Amplifier Puts Programmable Gain Anywhere by Max W. Hauser Introduction 50 V+ 2.7V TO 10V 0.1µF LTC6910-1 is programmable for eight gain magnitudes of 0, 1, 2, 5, 10, 20, 50, or 100 Volts/Volt—much like a classic oscilloscope amplifier plug-in or lab amplifier with a gain knob, but much smaller. This tiny DC-coupled, low-noise, self-contained amplifier, useful to low Megahertz frequencies, replaces expensive combinations of op amps and resistor arrays. The 8lead TSOT-23 package needs no other analog components. The LTC6910-1 operates from single or dual supplies, 2.7V to 10.5V total, and generates its own ground reference for use with single supplies. A simple 3-bit CMOS digital input controls the gain. The LTC6910-1 is an inverting voltage amplifier with a rail-to-rail output. At gains of unity and zero (digital inputs 001 and 000) it handles railto-rail input signals. At gains above unity (digital input 010 through 111), the input-referred noise decreases with increasing gain, as desired in a variable-gain amplifier for a wide range of input levels. When set for a gain of zero (digital input 000), the output remains active (tracking the analogground or AGND pin), but feedthrough from the signal input pin is low, typi- 40 30 GAIN OF 50 (DIGITAL INPUT 110) 4 3 LTC6910-1 5 7 6 1 VOUT = GAIN • VIN 2 GAIN (dB) 8 VIN VS = ±5V, VIN = 5mVRMS GAIN OF 100 (DIGITAL INPUT 111) GAIN OF 20 (DIGITAL INPUT 101) 20 GAIN OF 10 (DIGITAL INPUT 100) 10 GAIN OF 5 (DIGITAL INPUT 011) AGND 1µF OR LARGER GAIN OF 2 (DIGITAL INPUT 010) G2 G1 G0 0 Figure 1. Single-supply programmable amplifier using LTC6910-1 cally –122dB at 20kHz and –100dB at 200kHz. Output noise in this zerogain setting is typically 5.8µVRMS in a 200kHz bandwidth, which is 116dB (19 equivalent bits) below the maximum signal output at a 10V power supply. The output will source or sink 10mA into a load, and is currentlimited at approximately 30mA. Typical standing supply current is 2mA at 2.7V and 3mA at 10V total VSUPPLY. Easy to Use The LTC6910-1 amplifier has only three analog signal pins: input, output, and an analog-ground reference (AGND), which can provide a halfsupply reference for single-supply applications (Figure 1). The other pins GAIN OF 1 (DIGITAL INPUT 001) –10 100 1k 10k 100k FREQUENCY (Hz) 1M 10M Figure 2. LTC6910-1 frequency response at non-zero gain settings are the power supply and the three digital input pins. These high-impedance CMOS digital inputs accept both rail-to-rail logic levels at any supply voltage, and 0V and 5V levels when the supply voltage is ±5V. Table 1 relates the 3-bit input code to the resulting voltage gain and other characteristics. (Other versions of the product are the LTC6910-2 with 0–64 binary gain code and the LTC6910-3 with 0–7 gain code.) Figure 2 shows the typical frequency response. Circuit Description Internally the LTC6910-1 includes an operational amplifier, switched Table 1. LTC6910-1 gain settings and properties Nominal Voltage Gain Maximum Input Signal (For Unclipped Output Signal) (VP-P) G2 G1 G0 Volts/Volt (dB) Dual 5V Supply Single 5V Supply Single 3V Supply Nominal Input Impedance (k ) 0 0 0 0 –120 10 5 3 (Open) 0 0 1 –1 0 10 5 3 10 0 1 0 –2 6 5 2.5 1.5 5 0 1 1 –5 14 2 1 0.6 2 1 0 0 –10 20 1 0.5 0.3 1 1 0 1 –20 26 0.5 0.25 0.15 1 1 1 0 –50 34 0.2 0.1 0.06 1 1 1 1 –100 40 0.1 0.05 0.03 1 16 Linear Technology Magazine • December 2002 DESIGN FEATURES G2 G1 G0 7 6 5 Table 2. Resistor values in LTC6910-1 Gain CMOS LOGIC IN 3 INPUT R ARRAY FEEDBACK R ARRAY – MOS-INPUT OP AMP V+ 10k 1 OUT + 10k V– 8 2 4 V+ AGND V– 6910 F03 Figure 3. Block diagram of LTC6910-1 resistors, and CMOS decoding logic to drive the switches (Figure 3). The gain code is always monotonic: an increase in the 3-bit binary number (G2 G1 G0) increases the gain between the IN and OUT pins. For single-supply applications, an internal matched pair of 10kΩ resistors at the AGND pin generates a convenient half-supply reference voltage for input and output. The user can override this built-in analog ground reference by tying the AGND pin to a system reference voltage (within the AGND voltage range specified in the data sheet). The AGND pin presents a nominal impedance of 5kΩ due to the internal resistor pair. Digital inputs (G2 G1 G0) control the input and –20 (THD + NOISE)/SIGNAL (dB) –30 feedback resistances in the closedloop amplifier. In the design of the LTC6910-1, the lowest noise with gain variation would be achieved by varying the input R array in Figure 3. That, however, would impose a 100:1 inputresistance range for the closed-loop amplifier with a 100:1 gain range. To avoid such a wide variation of input resistance, logic in the LTC6910-1 trades off changing the input resistance and the feedback resistance (Table 2). This gain-control approach still produces near-minimal noise. When the gain setting is zero (digital input 000), switches disconnect the IN pin internally and short the feedback path in Figure 3 to reduce signal feedthrough and noise. –60 0 10k 10k 2 5k 10k 5 2k 10k 10 1k 10k 20 1k 20k 50 1k 50k 100 1k 100k Bandwidth of the LTC6910-1 depends on gain setting. The lower gain settings of 1, 2, and 5V/V (digital inputs 001–011) exhibit –3dB corner frequencies respectively of 7, 5, and 2.5MHz at ±5V supply (Figure 2). The gain-control strategy described above causes the gain settings from 10 to 100 (digital inputs 100–111) to show a different high-frequency response, with a constant gain-bandwidth product of approximately 11MHz. Figure 4 is a SINAD curve showing signal output more than 100dB above combined noise and distortion, with large-signal outputs and a ±5V supply. Applications Expanding an ADC’s Dynamic Range Figure 5 shows a compact data-acquisition system for wide-ranging input levels, which combines an LTC6910-1 programmable amplifier in an 8-lead TSOT -23 with an LTC1864 analog-to-digital converter 1µF 5V 0.1µF GAIN SETTING = 10 LTC1864 8 –70 4 –80 VIN 3 LTC6910-1 –90 6 –100 2 GAIN SETTING = 1 –110 0.01 ∞ 1 5V GAIN SETTING = 100 0.1 1 INPUT VOLTAGE (VP-P) RFB 0 fIN = 1kHz VS = ±5V NOISE BW = 22kHz –40 –50 RIN 7 1 499Ω 5 270pF VREF VCC IN+ SCK IN– SDO GND CONV AGND 10 Figure 4. LTC6910-1 THD plus noise, referred to the signal output Linear Technology Magazine • December 2002 1µF GAIN CONTROL ADC CONTROL Figure 5. Expanding an ADC’s dynamic range 17 DESIGN FEATURES R2 C2 – VIN C1 R1 – R – + R + – + GAIN CONTROL PGA (GAIN A) + 2π VOUT = (GAIN A)VIN BANDWIDTH CONTROL PGA (GAIN B) MID-BAND-GAIN = (GAIN A) 1 ≤ BANDWIDTH ≤ 2πR1C1 VOUT 6910 F05 1 R2 C2 (GAIN B) Figure 6. Low-noise AC amplifier with gain and bandwidth control (ADC) in an 8-lead MSOP. The LTC1864 ADC has 16-bit resolution and a maximum sampling rate of 250ksps. The LTC6910-1 expands the ADC’s input amplitude range by 40dB while operating from the same single 5V supply. The 499Ω resistor and 270pF capacitor couple cleanly between the LTC6910-1’s output and the switched-capacitor input of the LTC1864. The two ICs shown in Figure 5 have similar distortion performance, with total harmonic distortion (THD) levels about –90dB at 10kHz and –77dB at 100kHz. At a gain setting of 10 in the LTC6910-1 (digital input 100) and a 250ksps sampling rate in the LTC1864, a 100kHz input signal at V+ V – 0.1µF 8 4 VIN 3 LTC6910-1 R2 15.8k C1 10µF R1 15.8k 1 C2 1µF 1 2 5 3 6 2 4 7 0.1µF 0.1µF V+ LT1884 – + V– V+ 0.1µF 8 7 + – 0.1µF V+ V – VOUT 8 4 R4 15.8k 3 LTC6910-1 5 1 5 6 6 R3 15.8k 2 7 0.1µF V– GAIN CONTROL GN2 0 0 0 1 1 1 1 GN1 GN0 0 1 1 0 1 1 0 0 0 1 1 0 1 1 BANDWIDTH CONTROL GAIN = 1 GAIN = 2 GAIN = 5 GAIN = 10 GAIN = 20 GAIN = 50 GAIN = 100 BANDWIDTH 1Hz TO 10Hz BANDWIDTH 1Hz TO 20Hz BANDWIDTH 1Hz TO 50Hz BANDWIDTH 1Hz TO 100Hz BANDWIDTH 1Hz TO 200Hz BANDWIDTH 1Hz TO 500Hz BANDWIDTH 1Hz TO 1000Hz BW2 BW1BW0 0 0 1 0 1 0 0 1 1 1 0 0 1 0 1 1 1 0 1 1 1 Figure 7. Practical low-noise AC amplifier with gain and bandwidth control 10 GN2 GN1 GN0 = 001 0 BW2 BW1 BW0 1 1 1 –10 GAIN (dB) –20 BW2 BW1 BW0 0 0 1 –30 –40 BW2 BW1 BW0 1 0 0 –50 –60 –70 –80 1 10 100 1k FREQUENCY (Hz) 10k Figure 8. Measured frequency responses for Figure 7 18 100k 60% of full scale shows a THD of –75dB from the combination. 10kHz input signals under the same conditions produce measured THD values around –87dB. Noise effects (both random and quantization) in the ADC are divided by the gain of the amplifier and combined with the amplifier’s noise when referred to VIN in Figure 5. Because of this, the circuit can acquire a signal that is 40dB down from full scale of 5VP–P with an SNR of over 70dB. Such performance from an ADC alone (110dB of useful dynamic range at 250ksps) would be prohibitively expensive today. Low Noise AC Amplifier with Programmable Gain and Bandwidth Analog data acquisition can exploit band-limiting as well as gain, to suppress unwanted signals or noise. Tailoring an analog front end to both the level and bandwidth of each source maximizes SNR. Figure 6 shows a block diagram and Figure 7 the practical circuit for a low-noise amplifier with gain and bandwidth independently programmable over 100:1 ranges. One LTC6910-1 controls the gain and another controls the bandwidth. An LT1884 dual op amp forms an integrating lowpass loop with capacitor C2 to set the programmable upper corner frequency. The LT1884 also supports rail-to-rail output swings over the total supply-voltage range of 2.7V to 10.5V. AC coupling through capacitor C1 establishes a fixed low continued on page 24 Linear Technology Magazine • December 2002 DESIGN FEATURES inactive for an amount of time longer than the watchdog time-out period, the WDO line falls, indicating a loss of the periodic input. Figure 8 demonstrates how the LTC2901 can be used to monitor a switching regulator’s activity. In this application, the 3.3V input, 1.8V output and feedback voltage to the LTC1772 regulator are supervised. Furthermore, if the load goes open circuit, the LTC1772 switches into Burst Mode® operation, reducing the duty cycle at the gate of M1. The pulse spacing exceeds the watchdog time-out period, and the watchdog output falls indicating the low-load condition. Power Supply Margin Testing with the LTC2902 In high reliability system manufacturing and test, it is desirable to verify the correct operation of electrical components at or below the rated power supply tolerance. The LTC2902 is designed to complement such testing in two ways. First, the reset disable pin (RDIS) can be pulled low which forces the RST output high. With RDIS low, moving supply voltages below threshold does not invoke the reset command during margining tests. The individual comparator outputs operate normally with RDIS high or low, allowing for individual supply monitoring. LTC6910-1, continued from page 18 frequency corner of 1Hz, which can be adjusted by changing C1. Alternatively, shorting C1 makes the amplifier DC-coupled. (When DC is not needed, however, the AC coupling suppresses low frequency noise and all amplifier offset voltages other than the low internally-trimmed LT1884 offset in the integrating amplifier, which is the second amplifier in Figure 6. If desired, another coupling capacitor in series with the input can relax the requirements on input DC level as well.) 24 10k Table 3. LTC2902 Tolerance Programming 6 5V 3.3V 2.5V 1.8V R1 59k 1% R2 40.2k 1% 4 14 V1 RST V2 T0 T1 7 T0 T1 Tolerance VREF 9 Low Low 5% 1.210V Low High 7.5% 1.178V High Low 10% 1.146V High High 12.5% 1.113V 3 8 V3 RDIS LTC2902-1 13 2 V4 COMP1 16 12 COMP2 VREF 1 COMP3 11 15 VPG COMP4 GND 10 CRT 5 CRT 47nF Figure 9. Quad supply monitor with asymmetric hysteresis metric hysteresis, having 5% tolerance when supplies are rising and 12.5% tolerance after all supplies have safely crossed their 5% thresholds. Conclusion The second way allows the user to provide more supply headroom by lowering the trip thresholds. Using the digital tolerance programming inputs (T0, T1), the global supply tolerance can be set to 5%, 7.5%, 10%, or 12.5% (Table 3). When using the positive or negative adjustable inputs in conjunction with tolerance programming, external resistors need only to be sized once, based on a 5% tolerance threshold. Once the external resistor dividers are set using the 5% tolerance thresholds, the thresholds for the other tolerance modes (7.5%, 10%, 12.5%) are automatically correct because the reference voltage (VREF) is scaled accordingly. Figure 9 shows how the LTC2902 can be configured for asym- One part can now satisfy most present and future supervisory needs. The LTC2900, LTC2901 and LTC2902 micropower quad supervisors provide the versatility, accuracy and reliability required in multi-voltage monitoring applications. Input supply combinations are programmable including positive and/or negative adjustable thresholds. The comparators are 1.5% accurate over temperature and feature built-in noise rejection. Reset logic is correct for VCC down to 1V, and is available with open-drain or push-pull outputs. Reset and watchdog times are user adjustable with external capacitors. Power supply margining features include real-time supply tolerance selection and an ondemand reset disable pin. Measured frequency responses (Figure 8) demonstrate bandwidth settings of 10Hz, 100Hz, and 1kHz (digital BW inputs of 001, 100, and 111, respectively) and unity gain in each case. By scaling C2, this circuit can serve other frequency ranges, such as a maximum of 10kHz with 0.1µF using LT1884 (gain-bandwidth product around 1MHz). Output signal-to-noise ratio measured with 10mVP–P input, gain of 100, and 100Hz bandwidth is 76dB; for 100mVP–P input, gain of 10, and 1000Hz bandwidth it is 64dB. Conclusion With a printed circuit footprint of only about 11mm 2 , the easy-to-use LTC6910-1 provides two decades of programmable DC or AC voltage gain. It can preamplify, drive loads, and introduce gain flexibility into spaces so small that, as one engineer put it, “your boss doesn’t even need to know it’s there.” Acknowledgements Mark Thoren and Derek Redmayne collaborated on the ADC application and Philip Karantzalis contributed the AC amplifier. Linear Technology Magazine • December 2002