The Circuit Design Blog

Low-Pass Filter With Very Low DC Offset

This filter’s unusually low DC offset is due to the use of an OPA380 at U1. While this amplifier is usually employed as a transimpedance amplifier, it can also be used as an inverting op amp. In this configuration it provides wide bandwidth with extremely low input offset voltage and drift.

The OPA380 is a monolithic 85MHz GBW CMOS op amp with an internal auto- zeroed integrator. This integrator forces the high-speed op amp input offset and drift to virtually zero.

This 100kHz “Low-Pass Filter With Very Low DC Offset” circuit is a two- pole multiple- feedback filter with a Butterworth response. This filter has a DC gain of 1V/V or 0dB. Above the  -3dB corner frequency its response is close to theoretical up to 10MHz; above this frequency the finite GBW of U1 prevents much additional filter rolloff. An OPA380 is not suitable for a Sallen- Key active filter; that topology requires an op amp to be used as a non-inverting amplifier. The OPA380’s non-inverting input is a very low bandwidth integrator input. (Circuit is created by Neil P. Albaugh, TI – Tucson )

“Low-Pass Filter With Very Low DC Offset” Circuit:

Online Simulation of the “Low- Pass Filter With Very Low DC Offset” Circuit

The great feature of the TINA circuit simulator that you can analyze this circuit immediately with TINACloud the online version of TINA. Of course you can also run this circuit in the off-line version of TINA.

Click here to invoke TINACloud and analyze the circuit, or watch our tutorial video!

You can send this link to any TINACloud customers and they can immediatelly load it by a single click and then run using TINACloud.

Michael Koltai
www.tina.com

Capacitance Bridge

This circuit below is a capacitance bridge; it detects the matching between a reference capacitance C1 and an unknown capacitance, Cx. It uses an instrumentation amplifier in an unusual topology– a sine wave drives the two input op amps’ non- inverting inputs (ordinarily the IA inputs) and the internal 25k feedback resistor of each op amp forces that sine wave to appear at the op amp’s inverting inputs (ordinarily the IA gain resistor connections). The impressed AC voltage across each capacitor causes current to flow in each feedback resistor and this creates a voltage at the output of each of the two input op amps. The third IA stage, a differential amplifier, subtracts the two voltages. Thus, when Cx = C1, the output voltage is zero.

A higher or lower capacitance at Cx unbalances the bridge

and an output results that is proportional to the capacitance difference; a high/low mismatch is indicated by a 180 degree phase difference.

A synchronous detector (aka phase- sensitive demodulator) driven by F and low- pass filtered will result in a capacitance bridge DC output that is at null when Cx = C1.  The sensitivity of the bridge is proportional to F, both in amplitude and frequency, and to the reference capacitance C1. Higher frequencies result in higher output voltages but the inevitable IA CMRR roll- off at high frequencies will reduce the depth of the null voltage.  An advantage of this circuit is that it is quite simple and it allows both capacitors to be ground- referenced.  (Circuit is created by Neil P. Albaugh,  TI-Tucson)

Capacitance Bridge circuit:

Online Simulation of the “Capacitance Bridge” Circuit

The great feature of the TINA circuit simulator that you can analyze this circuit immediately with TINACloud the online version of TINA. Of course you can also run this circuit in the off-line version of TINA.

Click here to invoke TINACloud and analyze the circuit, or watch our tutorial video!

You can send this link to any TINACloud customers and they can immediatelly load it by a single click and then run using TINACloud.

Michael Koltai
www.tina.com

Triangle & Square Wave Oscillator 1kHz

This simple relaxation oscillator provides both a square- wave and a triangular- wave output. This oscillator is biased for operation on a single +5V supply. The DC component of each output can be removed by capacitive coupling if necessary.Since a rail-to-rail output op amp is used for U1, the square wave output amplitude is the same as its supply voltage. A lower quiescent current op amp such as an OPA364 can be used if Iq is important. Oscillator frequency is determined by C1 & R1. The Transient Analysis used the “Zero initial conditions” to aid the oscillator start- up. This start- up time is visible for the first few milliseconds in the waveform above. (Circuit is created by Neil P. Albaugh  TI- Tucson)

“Triangle & Square Wave Oscillator 1kHz” circuit:

Online Simulation of the “Triangle & Square Wave Oscillator 1kHz” Circuit

The great feature of the TINA circuit simulator that you can analyze this circuit immediately with TINACloud the online version of TINA. Of course you can also run this circuit in the off-line version of TINA.

Click here to invoke TINACloud and analyze the circuit, or watch our tutorial video!

You can send this link to any TINACloud customers and they can immediatelly load it by a single click and then run using TINACloud.

Michael Koltai
www.tina.com

Programmable-Brightness LED Control

This simple op amp circuit can be used to control the brightness of a LED. By placing the LED in the op amp feedback, it is driven  in a constant- current mode. This eliminates the diode forward voltage temperature coefficient’s effect on its current and thus its  brightness over temperature.  This circuit is a voltage- controlled current source. (Circuit is created by Neil P. Albaugh,  TI – Tucson)

Programmable-Brightness LED Control circuit:

Online Simulation of the Programmable-Brightness LED Control Circuit

The great feature of the TINA circuit simulator that you can analyze this circuit immediately with TINACloud the online version of TINA. Of course you can also run this circuit in the off-line version of TINA.

Click here to invoke TINACloud  and analyze the circuit, or watch our tutorial video!

You can send this link to any TINACloud customers and they can immediatelly load it by a single click and then run using TINACloud.

Michael Koltai
www.tina.com

Instrumentation Amplifier Offset Correction Loop

The feedback from integrator U2 provides a DC offset nulling function to the instrumentation amplifier (IA) U1. Although the IA response is similar to an AC- coupled amplifier, its input is, in fact, still DC- coupled and its input common-mode voltage limits must be observed.

Dc response can be preserved if a switch is added in series with R1. With the switch momentarily closed, the loop error is nulled and stored on C1 when the switch is open.
The switch converts the integrator into a sample/hold amplifier. To minimize correction voltage droop due to bias current, a JFET op amp such as an OPA132 is recommended for S/H use. Bypass capacitors are not shown. (Circuit is created by Neil P. Albaugh,  TI – Tucson)

Instrumentation Amplifier Offset Correction Loop circuit:

Online Simulation of the Instrumentation Amplifier Offset Correction Loop Circuit

The great feature of the TINA circuit simulator that you can analyze this circuit immediately with TINACloud the online version of TINA. Of course you can also run this circuit in the off-line version of TINA.

Click here to invoke TINACloud  and analyze the circuit, or watch our tutorial video!

You can send this link to any TINACloud customers and they can immediately load it by a single click and then run using TINACloud.

Michael Koltai
www.tina.com