Would appreciate some tips on designing simple OpAmp circuit

Question

I've recently posted this question regarding input impedance when working with DC signal on HCPL-7840 Isolation Amplifier. The answers were very clarifying and helpful, but I've come to realize I need more specific help.

Just a disclaimer: I decided to use HCPL-7840 because it was the best option available on my chosen supplier that seems to fufil my needs.

My goal is to design a simple isolated amplifier for an NTC

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• NTC characteristics considered:

  Resistance [kΩ]	Temperature [ºC]
  10	30
  15	20
  20	10
  25	0

• HCPL-7840 characteristics considered:

  • The information on the "Recommended Operating Conditions" table of datasheet
  • Equivalent Input Impedance RIN — 500 — kΩ
  • Figures 12 and 14 of datasheet

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• Based on these informations, I designed the following simple circuit:
  • It's composed of a simple voltage divider, where R1 is fixed precise resistor, and R2 is the NTC.

• As soon as I assembled the board and started making measurements, I noticed that the voltages on TP1 were not exactly what I expected to see

  • I was expecting a simple voltage divider between R1, R2
  • I figured the difference should be because of internal resistence of HCPL (Rin), between pins 2 and 3
  • This Rin is suposelly connected in parallel with R2 (between TP1 and GND iso)

• I than started to conduct some experiments to empirically try and figure out the value of Rin. This way I could calculate adjusted values for R1 and R2 and keep the voltage on TP1 within the input limits of HCPL linear range

  • The experiment consists of trying different combinations of R1 and R2 and measuring the voltage at TP1.

  • The results of this experiments drive me crazy because different combinations of R1, R2 make me infer different values for Rin each time.

  • The following table summarizes the experiment results:

    Index	Vdd [V]	R1 [Ω]	R2 [Ω]	V measured @ TP1 [V]
    1	5.40	2150	100	0.213
    2	5.45	2150	68	0.158
    3	5.56	2150	40	0.101
    4	5.55	246000	11600	0.186
    5	5.70	246000	5800	0.119
    6	5.77	717500	11600	0.082
    7	5.79	717500	23500	0.113
    8	5.84	717500	51300	0.142

  • Vdd and V measured @ TP1 were measured with osciloscope.

  • R1 and R2 were measured with Ohmmeter before the experiment.

  • Outcomes:

    • Given Vdd, R1 and V measured @ TP1, I was able to calculate what would be the "real R2" that the circuit is submited to.
    • This "real R2" would be the parallel between R2 (from the table) and Rin
    • Because I have R2, I'm able to calculate Rin

  • After this procedure I ended up with a lot of different values of Rin for each experiment case, and I don't know how to proceed!!!

Any help is appreciated.

Thanks!

Answer 1

Figure 14 of the datasheet suggests that the input current into the HCPL-7840 should be fairly linear (resistor like) for input voltages in the range of -200mV to +200mV. But acconding to note 12 on page 18 of the datasheet the input stage is actually a "switched capacitor", not a real resistor, so there is going to be current spikes. I would suggest putting a 0.1uF ceramic capacitor across R2.

I would also suggest making changes that make your circuit less sensitive to input current on the HCPL-7840 input pins.

• One option is to buffer the + input of the HCPL-7840 with another rail-to-rail op-amp configured as a voltage follower.
• Alternatively, you could lower the values of R1 and R2 enough that the input current into the HCPL-7840 doesn't affect the measurement as much.

Answer 2

from datasheet ...

Description
The Broadcom® HCPL-7840 isolation amplifier family is designed for current sensing in electronic motor drives. In a typical implementation, motor currents flow through an external resistor and the resulting analog voltage drop is sensed by the HCPL-7840. A differential output voltage is created on the other side of the HCPL-7840 optical isolation barrier. This differential output voltage is proportional to the motor current and can be converted to a single-ended signal by using an op-amp as shown in the recommended application circuit.

You need using it as a "differential" device. Not single input(ok)/output(not ok).
So you need using something as figure 4 in the datasheet.