# applications of precision full wave rectifier

A reader has since pointed out something I should have seen (but obviously did not) - R3 should not be installed. The opamps used must be rail-to-rail, and the inputs must also accept a zero volt signal without causing the opamp to lose control. For a low frequency positive input signal, 100% negative feedback is applied when the diode conducts. Similar circuitry can be used to create a precision full-wave rectifier circuit. Minimum suggested input voltage is around 100mV peak (71mV RMS), which will give an average output voltage of 73mV. A full wave precision rectifier can be made also by using a diode bridge. This applies to most of the other circuits shown here as well and isn't a serious limitation. During the positive cycle of the input, the signal is directly fed through the feedback network to the output. There is no output voltage as such, but the circuit rectifies the incoming signal and converts it to a current to drive the meter. Precision Rectifier using LT1078. The impedance presented to the driving circuit is very high for positive half cycles, but only 10k for negative half-cycles. Mobile phones, laptops, charger circuits. Nominal gain as shown is 1 (with R3 shorted). The above circuits show just how many different circuits can be applied to perform (essentially) the same task. This type of circuit almost always has R2 made up from a fixed value and a trimpot, so the meter can be calibrated. Full-wave Precision Rectifiers circuit . The output of the rectifier is processed further in the BA374 circuit to provide a logarithmic response which allows the meter scale to be linear. This rectifier is something of an oddity, in that it is not really a precision rectifier, but it is full wave. In most cases it is not actually a problem. R1 can be duplicated to give another input, and this can be extended. Uninterruptible Power Supply (UPS) circuits to convert AC to DC. This dual-supply precision full-wave rectifier can turn The Full Wave Bridge Rectifier Circuit is a combination of four diodes connected in the form of a diamond or a bridge as shown in the circuit. Additional weaknesses may show up in use of course. Where a simple, low output impedance precision rectifier is needed for low frequency signals (up to perhaps 10kHz as an upper limit), the simplified version above will do the job nicely. TI Precision Designs are analog solutions created by TI’s analog experts. The circuit shown figure 7.2.4 is an absolute value circuit, often called a precision full-wave rectifier. At input voltages of more than a volt or so, the non-linearities are unlikely to cause a problem, but diode matching is still essential (IMO). When the input Vin exceeds Vc (voltage across capacitor), the diode is forward biased … To understand the reason, we need to examine the circuit closely. The circuit will always have more or less the same input voltage, and voltage non-linearity isn't a problem. This circuit gives an overview of the working of a full-wave rectifier. These both have the advantage of a lower forward voltage drop, but they have higher reverse leakage current which may cause problems in some cases. User guide (2) Title Type Size (KB) Date ; Precision Full-Wave Rectifier, Dual Supply Design Guide; PDF: 1016: 08 Jan 2014 Peak detector. Limitations:   Linearity is very good, but the circuit requires closely matched diodes for low level use because the diode voltage drops in the first stage (D1 & D2) are used to offset the voltage drops of D3 & D4. The large voltage swing is a problem though. 18.9.4 Precision Full-Wave Rectiﬁer We now derive a circuit for a precision full-wave rectifier. The inverting input is of no consequence (it is a full wave rectifier after all), but it does mean that the input impedance is lower than normal ... although you could make all resistor values higher of course. The simplified version shown above (Figure 6) is also found in a Burr-Brown application note [ 3 ]. Digital meters have replaced it in most cases, but it's still useful, and there are some places where a moving coil meter is the best display for the purpose. Remember that all versions (Figures 7, 8 & 9) must be driven from a low impedance source, and the Figure 7 circuit must also be followed by a buffer because it has a high output impedance. This type of rectifier circuit is discussed in greater detail in AN002. It is simple, has a very high (and linear) input impedance, low output impedance, and good linearity within the frequency limits of the opamps. Full-Wave Rectifier with the transfer characteristic Precision Bridge Rectifier for Instrumentation Applications Figure 2 - Rectified Output and Opamp Output. The circuit is interesting for a number of reasons, not the least being that it uses a completely different approach from most of the others shown. Circuit modifications that help to meet alternate design goals are also discussed. In its simplest form, a half wave precision rectifier is implemented using an opamp, and includes the diode in the feedback loop. This knowledge applies to all subsequent circuits, and explains the reason for the apparent complexity. Without R6, the loading on D2 is less than that of D1, causing asymmetrical rectification. A forward voltage difference of only 10mV between any two diodes will create an unacceptable error. This circuit can be useful for instrumentation applications because it can provide a balanced output (on R L ) and, also a relative accurate high-input impedance. It can be done, but there's no point as the circuit would be far more complex than others shown here. The below circuit is non-saturating half wave precision rectifier. Figure 6A - Another Version of the AD Circuit. This general arrangement is (or was) extremely common, and could be found in audio millivoltmeters, distortion analysers, VU meters, and anywhere else where an AC voltage needed to be displayed on a moving coil meter. The additional diode prevents the opamp's output from swinging to the negative supply rail, and low level linearity is improved dramatically. If a 1V RMS sinewave is applied to the input, the meter will read the average, which is 900µA. Which we can create it by connecting the half-wave rectifier circuits together. As the efficiency of rectification is high in this rectifier circuit, it is used in various appliances as a part of the power supply unit. A center tap full wave rectifier has only 2 diodes where as a bridge rectifier has 4 diodes. The only restriction is that the incoming peak AC signal must be below the supply voltage (typically +5V for the OPA2337 or OPA2340). The Figure 6A version is also useful, but has a lower input impedance and requires 2 additional resistors (R1 in Figure 6 is not needed if the signal is earth referenced). This version is interesting, in that the input is not only inverting, but provides the opportunity for the rectifier to have gain. The first stage allows the rectifier to have a high input impedance (R1 is 10k as an example only). It was pointed out in the original application note that the forward voltage drop for D2 (the FET) must be less than that for D1, although no reason was given. Linsley-Hood, Wireless World, May 1981, Applications of Operational Amplifiers, Third Generation Techniques - Jerald Graeme, Burr-Brown, 1973, pp. Simple capacitor smoothing cannot be used at the output because the output is direct from an opamp, so a separate integrator is needed to get a smooth DC output. Figure \(\PageIndex{14}\): Precision full-wave rectifier. Although the waveforms and tests described above were simulated, the Figure 6 circuit was built on my opamp test board. There will be no loss in the input voltage signal. There are exceptions of course. The R/C network (R6, R7 and C1) sets the ballistics of the meter, which is determined by the attack and release times. This assumes a meter with a reasonably low resistance coil, although in theory the circuit will compensate for any series resistance. To overcome the voltage drop we use a precision rectifier circuit.