Datasheet AD215 (Analog Devices) - 9

AD215 Compensating the Uncommitted Input Op Amp GAIN AND OFFSET ADJUSTMENTS
The open-loop gain and phase versus frequency for the uncom-
General Comments
mitted input op amp are given in Figure 14. These curves can The AD215 features an output stage TRIM pin useful for zero- be used to determine appropriate values for the feedback resis- ing the output offset voltage through use of user supplied circuitry. tor (RF) and compensation capacitor (CF) to ensure frequency When gain and offset adjustments are required, the actual com- stability when reactive or nonlinear components are used. pensation circuit ultimately used depends on the following:
25 80
• The input configuration mode of the isolation amplifier (non- inverting or inverting).
20 100 15 120
• The placement of any adjusting potentiometer (on the
PHASE
isolator’s input or output side).
10 140
As a general rule:
5 160 GAIN
• Gain adjustments should be accomplished at the gain-setting
0 180
resistor network at the isolator’s input.
–5 200
• To ensure stability in the gain adjustment, potentiometers
–10 220
should be located as close as possible to the isolator’s input
Ø, EXCESS PHASE – Degrees AVERAGE VOLTAGE GAIN – dB –15 240
and its impedance should be kept low. Adjustment ranges
–20 260
should also be kept to a minimum since their resolution and
–25 280
stability is dependent upon the actual potentiometers used.
100k 1M 10M 100M FREQUENCY – Hz
• Output adjustments may be necessary where adjusting poten- tiometers placed near the input would present a hazard to the Figure 14. Open-Loop Gain and Frequency Response user due to the presence of high common-mode voltages dur- ing the adjustment procedure.
Inverting, Summing or Current Input Configuration
Figure 14 shows how the AD215 can measure currents or sum • It is recommended that input offset adjustments are made currents or voltages. prior to gain adjustments. • The AD215 should be allowed to warm up for approximately
FB
10 minutes before gain or offset adjustments are made.
4 CF RF 47pF Input Gain Adjustments for Noninverting Mode IN– 3 OUT HI
Figure 16 shows a suggested noninverting gain adjustment cir-
IN+ OUTPUT 38 1 R R S2 S1 I FILTER, S
cuit. Note that the gain adjustment potentiometer RP is incorpo-
BUFFER V V
rated into the gain-setting resistor network.
S2 S1 AND TRIM IN COM OUT LO 2 CIRCUITRY 37 TRIM RIN = 2k
Ω
36 IN+ 1 AD215 OUT HI COM IN– 38 43 3 PWR RP OUTPUT C RTN F FILTER, 0.47pF R BUFFER C FB V AND SIGNAL 4 TRIM
Figure 15. Noninverting Summing/Current Configuration
RF RG IN COM CIRCUITRY OUT LO 2 37
For this circuit, the output voltage equation is:
TRIM 36
V
AD215 COM
O = –RF × (IS + VS1/RS1 + VS2/RS2 + . .)
43
where:
PWR RTN
V = Output Voltage (V) VS1 = Input Voltage Signal 1 (V) Figure 16. Gain Adjustment for Noninverting Configuration VS2 = Input Voltage Signal 2 (V) I For a ± 1% trim range: S = Input Current Source (A) RF = Feedback Resistor (Ω) (10 kΩ, typ) × RF R (R ≈1kΩ), R ≈ 0.02 × RG S1 = Input Signal 1 Source Resistance (Ω) P C R + G RF R S2 = Input Signal 2 Source Resistance (Ω) The circuit of Figure 15 can also be used when the input signal is larger than the ±10 V input range of the isolator. For example, in Figure 15, if only VS1, RS1 and RF were connected as shown with the solid lines, the input voltage span of VS1 could accom- modate up to ± 50 V when RF = 10 kΩ and RS1 = 50 kΩ. –8– REV. 0