Toubleshooting Buffered Enhanced Howland Current Pump Circuit - Page 1
Forum section: electronics / projects

author: Raj74
author_rank: Newbie
timestamp: July 25, 2023, 06:39:25 pm
content: Hi...new user here. I'm currently designing a bioimpedance measurement device using the AD5933 impedance analyzer IC which required additional analog front-end circuitry.I needed a constant current source for load excitation for which I chose the buffered enhanced Howland current topology. The output current requirement is under 300 uA. Input voltage is a sine wave of varying frequency with a 1.5 V dc offset. I am using the OPA2388 dual op amp IC with 0.05% resistors to make the current source. The supply voltage is 3 V and the ADG849 switch is used to switch between the load and a calibration resistor. The simulations I ran in LTspice work just fine but when I made the circuit, it does not perform as expected. According to the simulations, the output current should be constant up to 10 kΩ load. The output current is not constant when load resistance is changed (I used several discrete resistors well under 10 kΩ). Regardless of the load resistance, the output of the main opamp is always saturated to Vcc and there is a current flow even when the input pin Vin is left open.I am afraid I might have overlooked some basic detail which is causing the issues. Can anyone suggest possible reasons why this circuit is not performing as expected or what areas I should investigate further? Any input is appreciated. The schematic (svg) file can be found here:PS - The PCB is supposed to go on top of a pre-existing device hence the unusual layout.

author: Terry Bites
author_rank: Super Contributor
timestamp: July 25, 2023, 07:51:11 pm
content: Although the opamp has a rail to rail common mode range, the differential amplifer at the heart of your circuit does not.The 1.5V bias puts it out of range imediately.See attached.As you are operating at LF, this may help.www.edn.com/op-amp-can-source-or-sink-current/I hope you're reading IEC60601 from cover to cover tonight.

author: Raj74
author_rank: Newbie
timestamp: July 28, 2023, 07:58:35 pm
content: Really appreciate you pointing me in the right direction. I got so fixated on the constant current source part that I lost sight of the bioimpedance safety standards. I was referencing the Circuit Note CN-0217 from analog devices which suggests to keep the whole signal chain at a dc bias of Vdd/2 for consistency. But as it turns out, using a dc biased excitation signal can cause polarization in cellular electrolytes which can skew impedance measurements. Also the safe current limit is below around100uA for frequency range of 100khz which is what I need.So I have updated my target specifications to reflect these facts. In literature, the output signal from AD5933 is filtered to remove the dc bias so that the signal to the current source is small amplitude AC signal centered around 0v. I simulated such and also implemented it using my lab power supply and the circuit worked!!! It was providing 50uA p-p current constantly throughout the impedance range. However, the new challenge I am facing is that since I need to drive the device using a rechargeable battery, operating the opamps in dual rail configuration will require additional circuitry which I don't have much experience with. Can I use something like LM2776 to get negative 3.3v from the Vout of arduino and use it in conjuction with the Vout to get +-3.3V rails or do I need to get a dual rail converter like LM27762??Thanks in advance.

author: Zero999
author_rank: Super Contributor
timestamp: July 28, 2023, 09:25:13 pm
content: Why not use an instrumentation amplifier IC for the Howland?That way you don't have to bother with precision matched resistors.https://www.ti.com/lit/ml/slyy054/slyy054.pdfLTSpice Simulation

author: Zero999
author_rank: Super Contributor
timestamp: July 29, 2023, 10:31:38 am
content: Quote from: Raj74 on July 25, 2023, 06:39:25 pmRereading the original post. You don't have enough voltage headroom for this to work properly. 300µA through 10k requires more than 3V, because there is some volt-drop across the sense resistor and output transistors.It works in LTSpice because SPICE models don't always emulate the output characteristics of amplifiers properly.